IBM 1130


The IBM 1130 Computing System, introduced in 1965, was IBM's least expensive computer at that time. A binary 16-bit machine, it was marketed to price-sensitive, computing-intensive technical markets, like education and engineering, succeeding the decimal IBM 1620 in that market segment. Typical installations included a 1 megabyte disk drive that stored the operating system, compilers and object programs, with program source generated and maintained on punched cards. Fortran was the most common programming language used, but several others, including APL, were available.
The 1130 was also used as an intelligent front-end for attaching an IBM 2250 Graphics Display Unit, or as remote job entry workstation, connected to a System/360 mainframe.

Description

The total production run of the 1130 has been estimated at 10,000.
The 1130 holds a place in computing history because it gave many people their first direct interaction with a computer. Its price-performance ratio was good and it notably included inexpensive, removable disk storage, with reliable, easy-to-use software that supported several high-level languages. The low price and well-balanced feature set enabled interactive "open shop" program development.
The IBM 1130 uses the same electronics packaging, called Solid Logic Technology, used in System/360. It has a 16-bit binary architecture, as do later minicomputers like the PDP-11 and Data General Nova.
The address space is 15 bits, limiting the 1130 to words of memory. The 1130 uses magnetic-core memory, which the processor addresses on word boundaries, using direct, indirect, and indexed addressing modes.

Models

IBM implemented five models of the 1131 Central Processing Unit, the primary processing component of the IBM 1130. The Model 1 through Model 5 describe the core memory cycle time, as well as the model's ability to support disk storage. A letter A through D appended to the model number indicates the amount of core memory installed.
IBM 1131 Central Processing Unit weighs about 760/1050 lb.
The Model 4 was a lower-priced product with a 5.6 µs cycle time. Some purchasers of performance upgrades observed that the field adjustment to achieve the improvement was surprisingly trivial.
The IBM 1132 printer relies on the 1130 processor rather than internal logic to determine when to fire the print wheels as they rotated. Printers for the Model 4 run more slowly, but the slower processor still can not keep up with it. The hardware manual discloses that when the Model 4 was servicing the two highest-level interrupts, it ran at the faster 3.6 µs cycle time. Some users of the Model 4 would write a phony printer driver that did not dismiss the printer interrupt, in order to benefit from the higher processor speed. However, lower-level interrupts are disabled during this interval, even the end-of-card interrupt from the 1442 card reader.
;Follow-on products
The IBM 1800 is a variant of the IBM 1130 for process control applications. It uses hardware rather than core memory for the three index registers and features two extra instructions plus extra interrupt and I/O capabilities. It is a successor to the IBM 1710, as the IBM 1130 is a successor to the IBM 1620.
The IBM 1500 is a multi-user educational system based around either an IBM 1130 or an IBM 1800. It supports up to 32 student work stations, each with a variety of audio-visual capabilities.
Other than these, IBM produced no compatible successor systems to the 1130. The IBM System/7 is a process control and real-time system, and the IBM Series/1 is a general-purpose 16-bit minicomputer.
;Chronology
To maximize speed and conserve space, the operating system and compilers are written entirely in assembly language and employ techniques that are rare today, including intermixing code and data as well as self-modifying code.
Much user programming was done in Fortran. The 1130 Fortran compiler can run on a machine with only 4,096 words of core—though the compiled program might not fit on such a machine. In this multi-pass compiler, each "phase" processes the entire source program and takes it another step toward machine code. For example, the first phase reads the source statements into memory, discards comment lines, removes spaces except in text literals, concatenates continuation lines and identifies labels. The compiler was available in a disk-resident version as well as on 8-channel punched paper tape or punched cards.
The most widely used operating system for the 1130 was the Disk Monitor System Version 2 introduced in 1967. DM2 is a single-task batch-oriented system. It requires a system with at least 4 KB of core memory and one integrated 2310 disk drive for system residence. The Supervisor is tiny by modern standards, containing assorted system details such as first-level interrupt routines, called Interrupt Level Subroutines, plus the disk driver and routines to load the interpreter of job control commands and the card reader driver. Device drivers for other I/O devices required by a job are incorporated as part of the loading of that job, which might also include the replacement of the basic disk driver by a more advanced driver. During the execution of a job, only a resident monitor, called the Skeleton Supervisor, resides in memory. This Supervisor requires just 1020 bytes, so a task's first available memory started with address /01FE or word 510. When the job ended or was aborted, the Supervisor loaded the Monitor Control Record Analyzer to read the job control for the next. While the job is running, the Supervisor is inactive. Aside from device drivers and interrupt processing all CPU time is entirely devoted to the job's activities. Other programs distributed as part of the operating system were a core dump utility, DUMP, and the Disk Utility Program, DUP.
A Card/Paper Tape Programming System was available to support systems without disk.
There is a hierarchy of device drivers: those ending in Z are for Fortran, such as DISKZ, while assembler programmers might use DISK0, and DISK1 was even faster at reading multiple disk sectors. But DISKZ starts its sector addressing with the first available unused sector, while the others start with sector zero of the disk, making it easy for a Fortran programmer dabbling in assembler to inadvertently overwrite the bootstrap loader.
Other programming languages available on the 1130 included
There is even an Algol compiler, written in French, so that "Debut...Fin;" take the place of "Begin... End;". All its messages were in French, so "Bonne compilation" is the goal.
Eastern Michigan University developed a Fortran IV compiler for the 1130, known as Fortran-EMU, as an alternative to the Fortran IV compiler provided by IBM. It adds many features, including the LOGICAL data type, enhanced diagnostics, and six-letter variable names. The Fortran-EMU compiler was distributed as a deck of punched cards in a disk image file format with all the remaining system area deleted, to prevent copying other modules that would normally reside on the same disk, such as the assembler or compilers.
Oklahoma State University developed an ALGOL 68 compiler, written in ANSI Fortran 1966.
A FOCAL interpreter was developed at the University of Michigan.
IBM also distributed a large library of programs, both IBM-supported and unsupported.
Since the 1130 was aimed primarily at the scientific market,
scientific and engineering programs predominated:
The 1130 also occupied a niche as a data processing machine for smaller organizations:
There is also special-purpose software:
The enduring memories of the IBM 1130 may have resulted from its need for continual human intervention. It was usually occupied running "jobs" specified by a deck of punched cards. The human operator would load jobs into the card reader and separate them back into jobs for return, perhaps along with printed output, to the submitter. The operator would also have to watch the 1130 for evidence of a malfunctioning or stalled job and intervene by pressing the key on the keyboard to skip ahead to the start of the next job.
Marking the start of a job was a punched card that started with // JOB. Any card that started with // was a command to the Supervisor and could not be used as user program or data. Other commands included // DUP to execute the Disk Utility Program and // XEQ to execute a named program from disk. If a user program tried to read a command card, the standard card reader routine would signal end-of-input to the program and save that card's content for the Supervisor.
;Initial Program Load
Unlike the IBM 360, where a booting device can be selected from the system console, an IBM 1130 can only be "booted" from an external device: a card reader or a paper tape reader.
The bootstrap procedure reads one card from the card reader. The boot card contains binary code to read the contents of sector zero of the disk drive, which in turn handles the "operation complete" interrupt from the disk drive and performs additional disk reads to prepare the 1130 for the first punched-card job. The whole process takes about a second to complete.
;Recovery procedures
When the IBM 1130 is started, the Supervisor is still in memory and probably intact, as core memory retains its state without power. If the operator concludes that a user program has stalled, the Supervisor can sense a key press to abort the program and skip ahead to the next // card. The Supervisor is not protected against modification by a badly written job, a case that might require that the operator reboot the 1130. Nor was there protection against writing to disk. If the copy of the system software on disk is modified, it can be restored by reloading it from about 4000 binary-coded punched cards.

Disk organization

The IBM 2310 disk drive stored sectors of 320 words plus a one-word sector address. A cylinder consisted of two tracks on the top and bottom surfaces of the 2315, or of one platter on the 1316 disk pack used in the 2311. Each disk cylinder contained eight sectors. A sector was logically divided by the monitor into sixteen disk blocks of 20 words each ; the disk block was the unit of allocation for files. The system distinguished between system cartridges, which contained the monitor and utilities along with user data, and nonsystem cartridges, which contained user data only. A system cartridge contained the cartridge id and the cold-start program in sector 0 followed by a communications area and the resident monitor in sectors one and two. Sectors three through five contained the System Location Equivalence Table —a directory of all phases of all monitor programs. Other control information filled out the first track.
The system area was followed by a fixed area containing system utilities, disk driver subroutines, IBM-supplied compilers and other control information. This area was mapped by a Fixed Location Equivalence Table containing the file format, file name, and disk block count. The fixed area also contained the Location Equivalence Table, in the same format as the FLET, mapping the following user area of the disk. The LET and FLET consisted of one entry per file on the disk giving the file's name and format information, its size in disk blocks, and its starting block number.
All disk files were contiguous disk blocks, thus there was no fragmentation. A program in need of working storage could use and modify named files, but could not expand them beyond their created size. Free space started after the last named file, and might be partially occupied by a temporary file, as the assembler or a compiler might produce. If a file was to be modified, the usual process was to use // DUP commands to delete it, which would move any subsequent files back to close the gap, and then give that name to the temporary file as the new version of the file. Rarely modified files would thus sink towards the start of the disk as new files or new versions were appended, and frequently modified files would jostle amongst each other towards the end of the disk.
Disk space was at a premium, so program source files were normally kept as decks of cards. Users having larger requirements would arrange to have a disk of their own containing the operating system but only their files and would have to replace the "pool" system disk with theirs and restart the system when their turn at being operator came. A system with a second disk drive that could be devoted entirely to some user's code and data provided a great sense of spaciousness.

Disk Utility Program (DUP)

The Disk Utility Program provided commands for transferring programs, subroutines and data. The "// DUP" card was followed by one or more cards:
The operands, as was typical back then, had to be placed into fixed/assigned columns. Aside from the names of programs, subroutines or files, codes/values could be:
Programs could be converted to a more ready-to-use format with the STORECI command, which invoked Core Image Builder. Alternatively, a program could go through this process each time it was to be run, and for infrequently used programs this was preferred in order to conserve disk space.

Peripheral devices

Disk memory was used to store the operating system, object code, and data, but source code was kept on punched cards.
The basic 1130 came with an IBM 2310 voice-coil actuated disk drive, called "Ramkit", from IBM's General Products Division in San Jose. Their pizza-box-sized IBM 2315 single platter cartridges held 512,000 words or 1,024,000 bytes.
The console typewriter used an IBM Selectric mechanism, which meant one could change the type by replacing a hollow, golf-ball sized type element. There was a special type element available for APL, a powerful array-oriented programming language using a special symbolic notation. A row of 16 toggle switches on the console typewriter could be individually tested from within programs, using the special Fortran statement IF , for example.
Other available peripherals included:
To simplify the design of peripheral devices, these relied on the processor. The card reader had no memory buffers, but instead gave the CPU a level-zero interrupt after each individual column of the card had been read. If the CPU did not respond and store the twelve bits of data before another such interrupt indicated that the next column had been read, data would be lost. Similarly, the 1132 printer relied on software on the 1130. When a letter such as A came into position, the CPU had to analyze a buffered line of text and assemble an array of bits that would indicate to the 1132 which print positions should be printed with A. If the CPU could not respond before the A rotated out of position, print speed would be severely degraded.
Other peripherals accepted text in a device-specific code convenient for its hardware. The CPU had to translate it to or from the EBCDIC code in which the CPU processed the text.

Instruction set overview

Instructions had short and long formats. Most computational, load, and store instructions referenced one register and a memory location. The memory location was identified, in the short format, by an 8-bit signed displacement from either the current address or one of the index registers; or in the long format, by a full 15-bit address, which could be indexed and specify indirection. Memory was addressed in units of words.
The 1130 supported only single-precision and double-precision binary data natively stored in big-endian format. Standard- and extended-precision floating-point and decimal data were supported through the use of subroutines.
Conditional transfers were based on the current contents of the accumulator, or the carry and overflow indicators set by a preceding operation. Transfers could be by skip or by branch. A skip occurred if any of the specified tests were true. A branch occurred if none of the specified tests were true.

Main Registers:
IAR = Instruction Address Register
ACC = Accumulator
EXT = Extension Register
XRx = Index Registers: x = 1,2,3
 Implemented as memory words 1,2,3, not as hardware registers.
Condition tests:
Z Accumulator zero
- Accumulator negative
+ Accumulator positive
E Accumulator even
C Carry indicator off
O Overflow indicator off
1130 Instruction Set Mnemonics:
LD = Load ACC STO = Store ACC
LDD = Load Double STD = Store Double 
LDX = Load Index STX = Store Index
LDS = Load Status STS = Store Status
A = Add ACC AD = Add Double
S = Subtract ACC SD = Subtract Double
M = Multiply D = Divide
AND = Boolean AND OR = Boolean OR
XOR = Boolean Exclusive OR
SLA = Shift Left ACC SLT = Shift Left ACC & EXT
SLCA = Shift Left and Count ACC SLC = Shift Left and Count ACC & EXT
SRA = Shift Right ACC SRT = Shift Right ACC & EXT
RTE = Rotate Right ACC & EXT
BSC = Branch or Skip on Condition 
 i.e. BP BNP BN BNN BZ BNZ BC BO BOD
BOSC - Branch Out or Skip Conditionally 
 Exits current interrupt level.
BSI = Branch and Store IAR
MDX = Modify Index and Skip 
WAIT = Halt NOP = No Operation 
XIO = Execute I/O
1800 Additional Instruction Mnemonics:
CMP = Compare ACC DCM = Double Compare ACC & EXT
Equivalent Mnemonics
The disk assembler introduced several mnemonics equivalent to existing
instructions intended to make the programmer's intent clearer:
SKP - Skip on condition, equivalent to a short BSC
B - Branch unconditionally, equivalent to BSC with no conditions specified
BP - Branch Accumulator Positive, equivalent to BSC specifying '+' condition
BNP - Branch Accumulator not Positive
BN - Branch Accumulator Negative
BNN - Branch Accumulator not Negative
BZ - Branch Accumulator Zero
BNZ - Branch Accumulator not Zero
BC - Branch on Carry
BO - Branch on Overflow
BOD - Branch Accumulator Odd
MDM - Modify Memory, equivalent to unindexed long-format MDX
XCH - Exchange Accumulator and Extension, equivalent to RTE 16
Short instruction format :
 1
Bits 0...45678......5
 OP---FTTDisp----
OP is Operation
F is format 0 = Short
TT is Tag
Disp is Displacement
Long instruction format :
 1 1
Bits 0...456789.....50..............5
 OP---FTTIMod----Address---------
OP is Operation
F is format 1 = Long
TT is Tag
I is Indirect bit
Mod is Modifier
Effective Address Calculation :
 F = 0 | F = 1, I = 0 | F = 1, I = 1
 Direct Addressing| Direct Addressing| Indirect Addressing
-------------------------------------------------------------------
TT = 00 | EA = Displ + IAR | EA = Add | EA = C/Add
TT = 01 | EA = Displ + XR1 | EA = Add + XR1 | EA = C/Add + XR1
TT = 10 | EA = Displ + XR2 | EA = Add + XR2 | EA = C/Add + XR2
TT = 11 | EA = Displ + XR3 | EA = Add + XR3 | EA = C/Add + XR3
-------------------------------------------------------------------
 Disp = Contents of displacement field
 Add = Contents of address field of instruction
 C = Contents of location specified by Add or Add + XR

;Reserved memory
The lowest addresses of core memory had uses dictated either by the hardware or by convention:
AddressUsage
/0000By convention, contained the instruction B *-1, which would branch to itself indefinitely until an operator noticed that all the console IAR lights were dark and flushed the job, either by pressing Int Req or by rebooting.
/0001XR1. The memory addresses of the index registers permitted direct moves between them, such as with LDX I1 2
/0002XR2.
/0003XR3.
/0008The address of the handler for the Level 0 interrupt— 1442 card reader/punch "column ready" interrupt.
/0009The address of the handler for the Level 1 interrupt— 1132 printer and Synchronous Communications Adapter. Handlers for this and lower interrupts would have to test a Status Word to determine which device had interrupted.
/000A=10The address of the handler for the Level 2 interrupt— disk storage, Storage Access Channel.
/000B=11The address of the handler for the Level 3 interrupt— 1627 plotter, Storage Access Channel.
/000C=12The address of the handler for the Level 4 interrupt— 1134 paper tape reader, 1055 paper tape punch, console, 1442 card read punch, 2501 card reader, 1403 printer, 1231 optical mark reader, Storage Access Channel device.
/000D=13The address of the handler for the Level 5 interrupt— console stop and interrupt switches, Storage Access Channel.
/0020=32First word of the scan field for the 1132 printer.
/0026=38Last full word of the scan field.
/0027=39Half used: 120 columns = 120 bits = seven 16-bit words plus 8 bits.
/0038=56EXIT to Supervisor/Return to Monitor

Programming

Subprograms

The 1130 has no hardware support for a stack. Most subprograms are called with the instruction BSI. This deposits the value of IAR at the destination address and transfers control to destination+1. Subprograms return to wherever they were called on that occasion using an indirect branch through that first word of the subprogram. Placing the return address in-line was a common technique of computers at that time, such as the Hewlett-Packard HP 2100 and the Scientific Data Systems SDS 920.
So a subprogram named SIMPL might be organized as follows :
SIMPL: DC *-* This is the entry point, filled with a zero initially.

B I SIMPL Return by an Indirect branch, to the address found in location SIMPL.
END SIMPL Instructs the assembler that the source for routine SIMPLE is complete.
The subprogram would be called as follows:
BSI L SIMPL Call SIMPL. L is needed if SIMPL is more than -128 or +127 words away.
The pseudo-operation CALL would typically be used.
As shown, a subprogram's entry point is DC *-*, an assembler pseudo operation that is used to Define a Constant with the value specified by the expression. The * stands for the current address of the assembly and so *-* results in zero. Writing this rather than 0 provides a visually distinctive note that a meaningful value will be placed there at run time. The entry point need not be the first word of the subprogram. Indeed, the preceding word can be the start of a two-word direct branch instruction whose address field is at SIMPL. Then, returns can be effected by one-word branches there: B SIMPL-1
When SIMPL is called, the BSI instruction replaces *-* with the current value of IAR, which is the address just past the BSI instruction. After SIMPL does whatever it is written to do, B I SIMPL branches not to SIMPL, but indirect through it, thus continuing execution with the instruction following the BSI instruction that called SIMPL.
Without extra arrangements to protect the return address, recursion is impossible: If SIMPL calls itself, or called a subprogram that called it, its original return address is overwritten. Re-entrancy is problematic for the same reason: An interrupt service routine must refrain from calling any subprogram that might have been the code that was interrupted.
The caller of SIMPL might pass it parameters, which might be values or addresses of values. Parameters might be coded in-line or might be placed in index registers XR1 and XR2. If parameters are placed in-line, SIMPL modifies its own return address so its final indirect branch returns beyond the parameters.
Integer functions of a single integer expect the parameter in the accumulator and return their result there. Floating-point functions employ the floating-point accumulator, and so on.
The convention of coding 0 as the initial value at the entry point means that if a programming error leads to SIMPL returning before the first time it was ever called, execution would jump to memory location 0. As mentioned [|above], it is customary to have location 0 contain a branch to location 0. The 1130 would be stuck at location 0, and the IAR lights on the console would be entirely dark, making it clear the program had failed.

Linkage to library routines

For subprograms that would be called many times, it is important to reduce the size of each call to one word. Such "library routines" use the LIBF protocol. It is more complex than the CALL protocol described in the previous section, but LIBF hides the complexity from the writer of the assembly-language program.
Library routines are addressed through index register XR3. XR3 points to a sequence of three-word transfer vectors such that the first entry is -128 words from XR3's value. The programmer calls the library routine using the LIBF pseudo-operation, which assembles not a direct BSI to the routine but a one-word indexed branch instruction whose displacement identifies the start of the routine's transfer vector.
The transfer vector is prepared by the linkage loader when it puts together the program. A transfer vector entry to a library function named SIMPL takes this form:
DC *-* A word into which BSI stores the return address.
B L SIMPL Branch to the start of the library function.
The way SIMPL knew where its return address was is that, if SIMPL were declared a LIBF routine, the linkage loader would modify the code of SIMPL, placing the address of SIMPL's transfer vector entry at SIMPL+2. LIBF routines, unlike CALL subprograms, do not start with a DC directive to hold the return address but with actual code, as follows:
SIMPL STX 1 RCVR1+1 Save the caller's value of XR1 at a nearby location.
LDX I1 *-* The linkage loader changes the address word to point to the transfer vector.
Placing the address of SIMPL's transfer vector at SIMPL+2 leaves room for a one-word instruction to save the chosen index register, here XR1. Then the indirect LDX instruction points XR1 not at the transfer vector, but through it to the return address, or to any parameters stored in-line after the BSI. SIMPL then does whatever it was written to do, gaining access to any in-line parameters through XR1, and returns as follows:
STX 1 RETN+1 Store XR1 to prepare to use it as a return address.
RCVR1 LDX L1 *-* SIMPL's first instruction modified this address. Now,
* restore the original value of XR1.
RETN B L *-* This instruction was modified two instructions ago; return.
;Example
Suppose a LIBF-style call to SIMPL were at address 100. Then the return address would be 101, because BSI 3 disp is a one-word instruction. XR3 points into the group of transfer vectors. If the transfer vector for SIMPL started at address 2000, then the BSI would be assembled with a disp so that XR3+disp = 2000. Executing the BSI stores 101 at location 2000 and jumps to location 2001. At 2001 is a two-word long jump to the entry point of SIMPL, which the linkage loader might have placed at address 300.
The long jump transfers control to SIMPL. After the instruction at 300 stores XR1, the instruction at 301 is LDX I1 2000, the linkage loader having placed 2000 at location 302. This does not load 2000 into XR1; it is an indirect instruction, and loads the contents of 2000, which is 101, the return address for that call to SIMPL.
In the return sequence shown above, by the time control reaches RETN, the instruction there is B L 101, which returns to the caller.
;Variations
If SIMPL took parameters coded in-line following the BSI instruction, SIMPL gains access to them with indexed addressing off XR1. The first could be obtained by LD 1 0, the second by LD 1 1, and so on. If the second parameter is the address of the actual parameter, then LD I1 1 obtains its value. Before returning, SIMPL increments XR1 past the n parameters with an instruction such as MDX 1 n so as to place the right value at RETN+1.
A LIBF routine that declined to restore the original value of XR1 could omit the above steps and return with a simple B 1 n to skip n in-line parameters. However, such a routine can not be called by other LIBF routines because it disrupts the caller's use of XR1 for access to its own parameters and return address.
The complexity of LIBF saves memory for subprograms that are frequently called.: The LIBF linkage requires one word per invocation, plus three words for the transfer vector entry and extra code in the routine itself, whereas the CALL linkage requires two words per invocation because most CALLs will be to an address beyond the -128 to +127 word reach of the one-word opcode.
The register XR3 must point to the transfer vector entries for the library routines rather than a dispatch table of only their addresses, because this latter would require that LIBF routines be called with an indirect BSI instruction. These instructions are two words long, so such a design would negate the code size savings of LIBF. The eight-bit limit for the disp field of the one-word instruction code limits usage of LIBF routines to no more than 85 distinct entries.

Code modification

The previous sections show that code and data are intermingled. It is common in 1130 programming to modify the address fields of instructions and, in fact, to modify entire instructions.

By the Fortran compiler

The Fortran compiler produces self-modifying code when generating code for any subprograms that have parameters. The compiler builds a table of every location where the subprogram references one of its parameters, and compiles as the first instruction in the body of the subprogram a call to a subprogram called SUBIN that uses the table to modify the address field of every reference to a parameter to be the actual address of the parameter during the current invocation. SUBIN makes these patches every time the subprogram is called.
When a Fortran program calls a subprogram, the addresses of any parameters appear in-line following the call. For example, the Fortran statement CALL SIMPL might compile into:
BSI L SIMPL
DC X The address of X, on which SIMPL is to operate
Within the subprogram, parameters could be accessed by indirect indexed addressing as shown above in Variations, so, given that XR1 has been suitably prepared, an integer parameter could be loaded into the accumulator with an instruction like this:
LD I1 0 Load the value of the first parameter into the accumulator
The compiler instead used direct addressing. When SUBIN runs, it obtains the address of X and patches the instruction's address field to become:
LD L X Load the value of X into the accumulator
The advantages of SUBIN are as follows:
The disadvantages of SUBIN are the time it requires to run and the memory required for the table of references. The size of this table is the sum of 5, the number of parameters, and the number of references; if this sum exceeds 511, compilation will fail. For subprograms with many references to a parameter, the author of the subprogram might copy the parameter into a local variable.

By the user

Modifying entire instructions was a common technique at the time. For example, although the 1130 has an OR instruction, the syntax of Fortran provides no way to write it. An integer function IOR can be defined, enabling logical OR to be part of a Fortran expression such as:

M = 3*IOR + 5

The Fortran compiler places the addresses of I and J in-line and expects the result in the accumulator. Using IOR in a Fortran expression compiles the following four words:
BSI L IOR Two-word jump to the start of the IOR function.
DC I A one-word in-line parameter: The address of I.
DC J A one-word in-line parameter: The address of J.
In fact, the assembler IOR function does not compute I or J at all. Instead, it replaces the above four words with the following:
LD L I Load accumulator with I
OR L J OR accumulator with J
After performing that transformation, it does not return past the end of the four-word block. Instead, it branches to the exact address from which it had been called originally. The BSI instruction is no longer there; what is now there is the two instructions it has just written. They combine the two integers with the machine-language OR instruction and leave the result in the accumulator, as required.
The call to IOR and the transformation of the four-word block happens at most once per program run. If the Fortran line illustrated above is executed again, it runs faster than it did the first time. Similar functions could be devised for other useful operations.
A function that self-modifies, as IOR does, can not be used in a Fortran subprogram on any of the parameters to that subprogram because it is incompatible with the SUBIN subprogram discussed above. IOR's transformation of its four-word calling sequence, shown above, moves the location of the address of variable I. On subsequent calls to the Fortran subprogram, the table of references to parameters would be in error and SUBIN would patch the wrong word, in this case placing the new address of I over the OR operation code.

Extended precision

1130 FORTRAN offered two floating point formats: a 32-bit "standard precision" format and a 40-bit "extended precision" format.
Standard precision format contains a 24-bit two's complement significand while extended precision utilizes a 32-bit two's complement significand. The latter format makes full use of the cpu's 32-bit integer operations. The characteristic in both formats is an 8-bit field containing the power of two biased by 128. Floating-point arithmetic operations are performed by software, and double precision is not supported at all. The extended format occupies three 16-bit words, with the extra space simply ignored.
The "*EXTENDED PRECISION" compiler option card tells the FORTRAN compiler to use 40 bits instead of 32 bits for Floating Point data.

Large Fortran programs

Data to be manipulated and the instructions that manipulate them have to reside together in core memory. The amount of installed memory is a key limitation. Fortran provides several techniques to write large programs despite this limitation.
;LOCAL subprograms
Fortran let any subprogram be designated as "LOCAL". Each LOCAL subprogram is an overlay; it is part of the disk-resident executable program but is only loaded into core memory during the time it is called. So, for example, six LOCAL subprograms would require only as much core memory as the largest, rather than the total amount for all six. However, none of the six can invoke another, either directly or through intermediary subprograms.
;Programs in phases
An entire Fortran program can pass control to a subsequent phase, exiting to the Supervisor with an instruction to load the follow-on phase into core memory. A large program might be split into three parts, separately compiled, called PART1, PART2, and PART3. Execution is started by // XEQ PART1 and at a suitable point, PART1 would execute the Fortran statement CALL LINK and so forth. The name of the successor program in the CALL can not be variable, but program logic can govern whether control is transferred to another phase, and which CALL LINK statement is executed. As mentioned above, the Fortran compiler itself was written this way, with each phase of compilation achieved by a separate program.
;COMMON data storage
Programs, such as Fortran programs, reside at low core memory addresses. Fortran allocates space at the highest addresses for any variables and arrays declared COMMON. If a follow-on phase of the program contains a corresponding COMMON declaration, then information in this common area can be shared among phases. Phases could omit the COMMON declaration without problem, provided those phases were not so large as to have their program code invade the common area. COMMON storage not only shares data between phases; lower-memory COMMON variables can be used to pass data among a main program and subprograms within a single phase, though the data could be lost on moving to the next phase.

Programming examples

The examples can be executed on the IBM 1130 emulator available at .

Sample assembler program deck

The following listing shows a card deck that compiles and runs an assembler program that lists a deck of cards to the line printer.

 // JOB
 // ASM
 *LIST
 * LCARD.ASM - LIST A DECK OF CARDS TO LINE PRINTER
 *
 * PROGRAM
 * NEW PAGE ON PRINTER
 * A READ A CARD
 * CONVERT FORMAT
 * PRINT A LINE ON PRINTER
 * GOTO A
 *
 START LIBF PRNT1 GOTO NEW PAGE ON 1132
 DC /3100 PRINTER CHANNEL 1-NEW PAGE
 *
 NEXTC LIBF CARD0 READ FROM 1442 CARD READER
 DC /1000 CONTROL TO READ
 DC CBUFF STORE 80 COLUMNS
 CINP LIBF CARD0
 DC 0
 B CINP LOOP UNTIL CARD IS READ
 *
 LIBF ZIPCO CONVERT CARD TO PRINTER
 DC /1100 UNPACKED IN, PACKED OUT
 DC CBUFF+1 INPUT BUFFER
 DC PBUFF+1 OUTPUT BUFFER
 DC 80 CHARACTER COUNT
 CALL HLEBC HOLLERITH TO EBCDIC
 *
 LIBF PRNT1 PRINT 80 CHARACTERS
 DC /2000 CONTROL CODE TO PRINT
 DC PBUFF PRINT BUFFER
 DC PERR PRINT ERROR
 POUT LIBF PRNT1 CHECK FOR PRINT COMPLETE
 DC 0
 B POUT LOOP UNTIL COMPLETE
 *
 B NEXTC READ NEXT CARD
 *
 * DATA
 *
 CBUFF DC 80 80 COLUMNS PER CARD
 BSS 80
 *
 PBUFF DC 40 40 WORDS 80 CHARACTERS
 BSS 40
 *
 PERR DC 0
 B I PERR THIS RETURNS TO THE
 * PRINTER ERROR HANDLER
 * WHICH WILL TERMINATE THE PROGRAM
 *
 END START PROGRAM ENTRY POINT
 // XEQ
 TEST DATA 1
 HELLO WORLD
 TEST DATA 2

In this job, the assembler leaves the result of its assembly in the temporary area of the system disk, and the XEQ command executes the content of the temporary area. The odd-looking END START has two meanings: end of assembler source, and the name of the entry point of the routine, which has the label START.
Assembler source starts with column 21 of the card, not column one. In systems without a disk drive, the assembler would punch code into the start of the card just read and then read the next card. To handle forward branches and the like, the assembler's second pass literally involved a second pass of the cards through the reader/punch. If source changes were needed the programmer would duplicate the cards to obtain a deck with columns 1-20 blank ready for the next run through the assembler.
By convention, buffers are preceded by a word count. The DC assembles a count word and the following BSS reserves the required number of words for the buffer. The card buffer requires 80 words, one for each card column. Driver CARD0 reads each card column literally, using 12 of the 16 bits in the buffer word, with a bit set to on for each hole punched in the corresponding row for that column. The pattern of punches typically describes a text character using the Hollerith code. The console keyboard also gives input to the program in the Hollerith code, the only case of two devices using the same character encoding.
The printer routine, however, works with text in 8-bit EBCDIC with two characters per word, requiring a 40-word buffer. The program uses library routine ZIPCO to perform the conversion. Despite appearances, the statement CALL HLEBC is not executed because HLEBC is not a subroutine but an IBM-supplied Hollerith-to-EBCDIC conversion table. The CALL statement provides the address of the table to ZIPCO and ensures that the linking loader includes the table in the program, thus it is the fifth parameter to ZIPCO, though one occupying two words of storage: the BSI operation code word for the CALL is unused and thus usually wasted, but the second word of the expansion of CALL HLEBC is the address of the HLEBC table needed by ZIPCO. After the conversion, the program sends the converted output, now in buffer PBUFF, to the printer through driver PRNT1. Again, the program loops until the printer driver reports completion, then the program reads the next card.
This example contains no code to decide when to stop. A more complete program would check for cards that begin with //, which denotes the start of the next job. To stop the card reader as soon as possible, a program could check for the Hollerith code of / before even converting the card to EBCDIC.

[Asynchronous I/O] and performance

The call to CARD0 to read a card initiates that operation and immediately returns to the caller, which could proceed with other activity. However, the example program makes no attempt to overlap input and output using buffers even though it has two separate work areas; it simply loops back to CIMP to test afresh. After CARD0 has sensed the card reader's operation-complete interrupt, it returns one word further on, thus skipping the jump back to CIMP and leaving the loop.
The example routines do not run the I/O devices at top speed. Notably, the card reader, only a few milliseconds after reporting completion on reading a card, will commence its stop sequence, after which a new read command will have to wait to initiate another read cycle. The IBM 1442 reader could read 400 cards/minute at full speed, but just a little hesitancy in the read commands would halve its throughput or worse. A Fortran program could not complete even the simplest input processing in time, and so could not read cards at full speed. One common Fortran DO loop to read cards made the motor stop and start so frequently as to accelerate wear. With buffering, the card reader control could be overlapped with processing, and the reader could be run at full speed through large data decks, but memory for the more complex program and for buffers was often at a premium.
Even with assembler and double buffering, a program to list a deck of cards from the IBM 2501 reader on the line printer could not keep up, as the translation from card hole patterns to EBCDIC for the printer as done by EBPRT was too slow; the more complex ZIPCO and HLEBC were needed instead, as in the example.

Sample Fortran IV program deck

The following listing shows a card deck that compiles and runs a Fortran program and Fortran subroutine for the IBM 1130 when running DM2.

// JOB
// FOR
*LIST SOURCE PROGRAM
*ONE WORD INTEGERS
C-------------------------------------------------------
C COMPUTE THE CRITICAL VALUES FOR A QUADRATIC EQN
C 0=A*X**2+B*X+C
C RETURNS DISCRIMINANT, ROOTS, VERTEX, FOCAL LENGTH, FOCAL POINT
C X1 AND X2 ARE THE ROOTS
C-------------------------------------------------------
SUBROUTINE QUADR
REAL A,B,C,DISCR,X1,X2,VX,VY,FL,FPY
C DISCRIMINANT, VERTEX, FOCAL LENGTH, FOCAL POINT Y
DISCR = B**2.0 - 4.0*A*C
VX = -B /
VY = A*VX**2.0 + B*VX + C
FL = 1.0 /
FPY = VY + FL
FL = ABS
C COMPUTE THE ROOTS BASED ON THE DISCRIMINANT
IF 110,120,130
C -VE DISCRIMINANT, TWO COMPLEX ROOTS, REAL=X1, IMG=+/-X2
110 X1 = -B /
X2 = SQRT /
RETURN

C ZERO DISCRIMINANT, ONE REAL ROOT
120 X1 = -B /
X2 = X1
RETURN
C +VE DISCRIMINANT, TWO REAL ROOTS
130 X1 = /
X2 = /
RETURN
C
C NEXT STORE SUBROUTINE ON DISK USING DUP
END
// DUP
*DELETE QUADR
*STORE WS UA QUADR
// JOB
// FOR
*LIST SOURCE PROGRAM
*IOCS
*ONE WORD INTEGERS
C-------------------------------------------------------
C PROCESS DATA CARDS WITH A,B,C
C UNTIL A=0
C-------------------------------------------------------
DATA ICARD,IPRT /2,3/
REAL A,B,C
REAL DISCR,XR1,XR2,VX,VY,FL,FPY
WRITE
901 FORMAT
C READ A B C, IF A=0 THEN EXIT
100 READA,B,C
801 FORMAT
C EXIT WHEN A IS ZERO
IF 110,9000,110
C PRINT A B C
110 WRITEA,B,C
902 FORMAT
C COMPUTE AND PRINT THE CRITICAL VALUES
CALL QUADR
WRITE DISCR
903 FORMAT
WRITE VX,VY
904 FORMAT
WRITE FL
905 FORMAT
WRITE VX,FPY
906 FORMAT
IF 120,130,140
C -VE DISCRIMINANT, TWO COMPLEX ROOTS
120 WRITE XR1, XR2
913 FORMAT
GO TO 200

C ZERO DISCRIMINANT, ONE REAL ROOT
130 WRITE XR1
912 FORMAT
GO TO 200
C +VE DISCRIMINANT, TWO REAL ROOTS
140 WRITE XR1, XR2
911 FORMAT
C --- GO TO 200
C END OF QUAD
200 WRITE
GO TO 100
C END OF PROGRAM
C DATA FOLLOWS XEQ CARD
9000 CALL EXIT
END
// XEQ
+001.000+000.000+000.000
+001.000+002.000+003.000
+002.000+002.000+000.000
+002.000+000.000-004.000
+000.500+000.000-004.000
+000.250+002.000-002.000
-004.000+000.000-004.000
+002.730-007.200-003.750
+000.000+000.000+000.000

Sample APL\1130 session

The following image shows a simple APL \ 1130 session. This session was performed via the 1130 simulator available from



The above session shows a signon, addition of the integers 1 to 100, generation of an addition table for the integers 1..5 and a sign off.

Competing systems

In the same year as the 1130's introduction, Digital Equipment Corporation introduced the smaller, cheaper, and better-selling 12-bit PDP-8, recognized as the first successful minicomputer.

Influence of the 1130

... I pounded the doors at the local IBM sales office until a salesman took pity on me. After we chatted for a while, he handed me a Fortran . I'm sure he gave it to me thinking, "I'll never hear from this kid again." I returned the following week saying, "This is really cool. I've read the whole thing and have written a small program. Where can I find a computer?" The fellow, to my delight, found me programming time on an IBM 1130 on weekends and late-evening hours. That was my first programming experience, and I must thank that anonymous IBM salesman for launching my career. Thank you, IBM.

The system was an IBM 1130 computer, a machine the size of a desk with 8 KB of main memory, a 512 KB disk drive, a Teletype CX paper tape reader and BRPE paper tape punch, and a Photon 713 photomechanical typesetter. The assignment was my first experience with managing a machine-readable document database: I learned to roll the punched paper tape carefully so that it could be stored neatly in cylindrical waste paper baskets.
In the meantime, though I didn't know about it, the roots of generalized markup were being planted. Historically, electronic manuscripts contained control codes or macros that caused the document to be formatted in a particular way. In contrast, generic coding, which began in the late 1960s, uses descriptive tags.

Out of an estimated 10,000 systems produced, the following are known to exist as of :
Speculation on why the product was given the number 1130 centered on the following possibilities:
Others have speculated that the existence of the IBM 1130 explains why no computer designated "11/30" ever appeared in the PDP-11 family of machines.