IBM Basic assembly language and successors

Assembler instructions, sometimes termed directives, pseudo operations or pseudoops on other systems, are requests to the assembler to perform various operations during the code generation process. For instance, CSECT means "start a section of code here"; DSECT provides data definitions for a structure, but generates no code; DC defines a constant to be placed in the object code.

One of the more important assembler instructions is USING, which supports the base-displacement addressing of the S/360 architecture. It guides the assembler in determining what base register and offset it should use for a relative address. In BAL, it was limited to the form

         USING base,reg-1,...,reg-n

Machine instruction addresses on S/360 specify a displacement (0–4095 bytes) from the value in a base register; while later versions of the architecture added relative-address formats, the older formats are still used by many instructions. USING allows the programmer to tell the assembler that the specified base registers are assumed to contain the address of "base", base+4096 (if multiple registers are specified), etc. This only provides a shortcut for the programmer, who otherwise would have to specify the base register in each instruction. Programmers are still responsible for actually loading the address of "base" into the register before writing code that depends on this value.

The related DROP assembler instruction nullifies a previous USING.

There is a one-to-one relationship with machine instructions. The full mnemonic instruction set is described in the Principles of Operation[4] manual for each instruction set. Examples:

 * This is a comment line
 * Load the fullword integer stored at the
 * location labeled 'ZIGGY' into general register 3:
       L     3,ZIGGY
       SLA   4,5             shift the value in general register 4 left by 5 bits
       MVC   TARGET,SOURCE   move characters from location 'SOURCE' to 'TARGET'
       AP    COUNT,=P'1'     add 1 to value in memory location 'COUNT' (packed decimal format)
       B     NEXT            unconditional branch to label 'NEXT'
 HERE  EQU   *               This is a label
       CLC   TARGET,=C'ADDRESS'  Compare memory location 'TARGET' to string 'ADDRESS'
       BE    THERE               branch if equal to program label 'THERE'

Generally accepted standards, although by no means mandatory, include the identification of general purpose registers with mnemonics. Unlike assemblers for some other systems, such as X86 assembly language, register mnemonics are not reserved symbols but are defined through EQU statements elsewhere in the program. This improves readability of assembler language programs and provides a cross-reference of register usage. Thus typically you may see the following in an assembler program:

 R3    EQU  3
       ...
       L    R3,ZIGGY

Some notable instruction mnemonics are BALR[lower-alpha 2] for a call storing the return address and condition code in a register, SVC,[lower-alpha 3] DIAG,[lower-alpha 4] and ZAP.[5]

System/360 machine instructions are one, two, or three halfwords in length (two to 6 bytes). Originally there were four instruction formats, designated by the first two bits of the operation code field; z/Architecture added additional formats.

The Basic Programming Support assembler did not support macros. Later assembler versions beginning with Assembler D[6] allow the programmer to group instructions together into macros and add them to a library, which can then be invoked in other programs, usually with parameters, like the preprocessor facilities in C and related languages. Macros can include conditional assembler instructions, such as AIF (an ‘if’ construct), used to generate different code according to the chosen parameters. That makes the macro facility of this assembler very powerful. While multiline macros in C are an exception, macro definitions in assembler can easily be hundreds of lines.

Most programs will require services from the operating system, and the OS provides standard macros for requesting those services. These are analogous to Unix system calls. For instance, in MVS (later z/OS), STORAGE (with the OBTAIN parameter) dynamically allocates a block of memory, and GET retrieves the next logical record from a file.

These macros are operating-system-dependent; unlike several higher-level languages, IBM mainframe assembly languages don't provide operating-system-independent statements or libraries to allocate memory, perform I/O operations, and so forth, and different IBM mainframe operating systems are not compatible at the system service level. For example, writing a sequential file would be coded differently in z/OS and in z/VSE.

The assembler for BPS is the true "basic assembler." It was intended to be loaded from cards and would run on an 8 KB System/360 (except Model 20). It has no support for macro instructions or extended mnemonics (such as BH in place of BC 2 to branch if condition code 2 indicates a high compare). It can assemble only a single control section and does not allow dummy sections (structure definitions). Parenthesized expressions are not allowed and expressions are limited to three terms with the only operators being '+', '-', and '*'.[7]^: 59–61

The Basic Operating System has two assembler versions. Both require 16 KB memory, one is tape resident and the other disk.[8]^: 7–8

Assembler D was the DOS/360 assembler for machines with a memory size of 16 KB. It came in two versions: A 10 KB variant for machines with the minimum 16 KB memory, and a 14 KB variant for machines with 24 KB. An F-level assembler was also available for DOS machines with 64 KB or more. D assemblers offered nearly all the features of higher versions.[9]^: 7

Assembler E was designed to run on an OS/360 system with a minimum of 32 KB of main storage, with the assembler itself requiring 15 KB.[10]^: 2 Assembler F can run under either DOS/360 or OS/360 on a system with a 64 KB memory, with the assembler requiring 44 KB.[11][12][13] These assemblers are a standard part of OS/360; the version that was generated was specified at system generation (SYSGEN).

Assembler H runs on OS/360 and successors; it was faster and more powerful than Assembler F, but the macro language was not fully compatible.

Assembler H Version 2 was announced in 1981 and includes support for Extended Architecture (XA), including the AMODE and RMODE directives.[14]^: 3-28 It was withdrawn from marketing in 1994 and support ended in 1995. It was replaced by High Level Assembler.[15]

Assembler XF is a mostly compatible upgrade of Assembler F that includes the new System/370 architecture instructions. This version provides a common assembler for OS/VS, DOS/VS and VM systems. Other changes include relaxing restrictions on expressions and macro processing. Assembler XF requires a minimum partition/region size of 64 KB (virtual). Recommended size is 128 KB.[16]^: 73

High Level Assembler or HLASM was released in June 1992 replacing IBM's Assembler H Version 2.[17][18] It was the default translator for System/370 and System/390, and supported the MVS, VSE, and VM operating systems. As of 2023 it is IBM's current assembler programming language for its z/OS, z/VSE, z/VM and z/TPF operating systems on z/Architecture mainframe computers. Release 6 and later also run on Linux, and generate ELF or GOFF object files (this environment is sometimes referred to as Linux on IBM Z).[19] While working at IBM, John Robert Ehrman created and was the lead developer for HLASM[lower-alpha 5] and is considered the "father of high level assembler".[21]

Despite the name, HLASM on its own does not have many of the features normally associated with a high-level assembler. The name may come from the additional macro language capabilities, such as the ability to write user-defined functions. The assembler is mostly similar to Assembler H and Assembler(XF), incorporating the SLAC (Stanford Linear Accelerator) modifications. Among features added were an indication of CSECT/DSECT for location counter, dependent[lower-alpha 6] and labelled[lower-alpha 7] USING statements, a list of USING statements currently active, an indication of whether a variable is read or written in the cross-reference, and allowing mixed-case symbol names.[22] The RSECT directive (Read-only Control Section) allows the assembler to check reentrancy on a per-section basis. RSECT was previously "undocumented and inconsistently implemented in Assembler H."[23]^: 41

The High Level Assembler Toolkit is a separately priced accompaniment to the High Level Assembler. The toolkit contains:[24]

• A set of structured programming macros —
  • IF/ELSE/ENDIF
  • DO/ENDDO
  • STRTSRCH/ORELSE/ENDLOOP/ENDSRCH
  • CASENTRY/CASE/ENDCASE
  • SELECT/WHEN/OTHRWISE/ENDSEL.
• A disassembler.
• A "Program Understanding Tool" (re-engineering aid).
• A Source XREF utility (cross-reference facility).
• Interactive Debug Facility.
• Enhanced SuperC (source comparison tool).

The IBM 7090/7094 Support Package, known as SUPPAK, "consists of three programs designed to permit programs written for a System 360 to be assembled, tested, and executed on an IBM 709, 7090, 7094, or 7094 II."

This cross-assembler runs on a 7090 or 7094 system and was used while System/360 was in development.[7][25] This assembler supports six-bit BCD character set as well as eight-bit EBCDIC.

IBM supplied two assemblers for the Model 20: the Model 20 Basic Assembler, and the Model 20 DPS/TPS Assembler. Both supported only instructions available on the Model 20, including unique instructions CIO, TIO, XIOB, SPSW, BAS, BASR, and HPR.[26]^: 110 The Basic Assembler is a slightly more restricted version of System/360 Basic Assembler;[27] notably, symbols are restricted to four characters in length. This version is capable of running on a system with 4 KB memory, and macro support is limited to IOCS macros. The card versions are two-pass assemblers that only support card input/output. The tape-resident versions are one-pass, using magnetic tape for intermediate storage. Programs assembled with the CPS Assembler can address a maximum of 16 KB.[27]^: 7–8

The DPS/TPS assembler is a somewhat restricted version of System/360 BPS/BOS Assembler.[26]^{: 132–134}

The IBM System/360 Model 44 Programming System Assembler processes a language that is a "selected subset" of OS/360 and DOS/360 assembler language.

Most significantly the Model 44 assembler lacks support for macros and continuation statements. On the other hand it has a number of features not found in other System/360 assemblers—notably instructions to update a card image source dataset, named common, and implicit definition of SETA assembler variables.[28]

It has no support for storage-to-storage (SS) instructions or the convert to binary (CVB), convert to decimal (CVD), read direct (RDD) and write direct (WRD) instructions.[29] It does include four instructions unique to the Model 44: Change Priority Mask (CHPM), Load PSW Special (LPSX), Read Direct Word (RDDW), and Write Direct Word (WRDW).

It also includes directives to update the source program, a function performed by utility programs in other systems (SKPTO, REWND, NUM, OMIT and ENDUP).[29]^: 53, 73

The assembler for the System/360 Model 67 Time Sharing System has a number of differences in directives to support unique TSS features. The PSECT directive generates a Prototype Control Section containing relocatable address constants and modifiable data used by the program.[30]^: 143

"Assembler G" is a set of modifications made to Assembler F in the 1970s by the University of Waterloo (Assembler F was/is open source). Enhancements are mostly in better handling of input/output and improved buffering which speed up assemblies considerably.[31] "Assembler G" was never an IBM product.