| Basic Assembly Language | |
|---|---|
| Paradigm | assembly language |
| Developer | IBM |
| First appeared | 1964 |
| OS | IBM Basic Programming Support,Basic Operating System/360 |
| License | free |
| Assembler D/E/F/G/H/XF | |
|---|---|
| Paradigm | macro assembly language |
| Developer | IBM |
| First appeared | 1966 |
| OS | IBMOS/VS,DOS/VS,VM/370 and related |
| License | varied |
| High Level Assembler | |
|---|---|
| Paradigm | macro assembly language |
| Developer | IBM |
| First appeared | 1992 |
| Stable release | Version 1 Release 6 |
| OS | IBMMVS/ESA and successors,VM/ESA and successors,VSE/ESA and successors |
| License | proprietary |
| Website | https://www.ibm.com/products/high-level-assembler-and-toolkit-feature |
| Majorimplementations | |
| High Level Assembler | |
TheIBM Basic assembly language and successors is a series ofassembly languages andassemblers made for theIBM System/360 mainframe system and its successors through theIBM Z.
The first of these, theBasic Assembly Language (BAL), is an extremely restrictedassembly language, introduced in 1964 and used on 360 systems with only 8 KB of main memory, and only acard reader, acard punch, and a printer forinput/output, as part ofIBM Basic Programming Support (BPS/360). The Basic Assembler for BAL was also available as part ofBasic Operating System/360 (BOS/360).
Subsequently, an assembly language appeared for the System/360 that had more powerful features and usability, such as support formacros. This language, and the line of assemblers that implemented it, continued to evolve for theSystem/370 and the architectures that followed, inheriting and extending its syntax. Some in the computer industry referred to these under the generic term "Basic Assembly Language" or "BAL".[1] Many did not, however, and IBM itself usually referred to them as simply the "System/360 Assembler Language",[2] as the "Assembler" for a given operating system or platform,[3] or similar names. Specific assemblers were known by such names[a] as Assembler E, Assembler F, Assembler H, and so forth. Programmers utilizing this language, and this family of assemblers, also refer to them as ALC (for Assembly Language Coding), or simply "the assembler".
The latest derived language is known as theIBM High-Level Assembler (HLASM).
As it is anassembly language, BAL uses the nativeinstruction set of the IBM mainframe architecture on which it runs,System/360, just as the successors to BAL use the native instruction sets of the IBM mainframe architectures on which they run, includingSystem/360,System/370,System/370-XA,ESA/370,ESA/390, andz/Architecture.
The simplicity of machine instructions means that thesource code of a program written in assembler will usually be much longer than an equivalent program in, say,COBOL orFortran. In the past, the speed of hand-coded assembler programs was often felt to make up for this drawback, but with the advent of optimizing compilers,C for the mainframe, and other advances, assembler has lost much of its appeal. IBM continues to upgrade the assembler, however, and it is still used when the need for speed or very fine control is paramount. However, all of the IBM successors to BAL have included a sophisticated macro facility that allows writing much more compact source code.
Another reason to use assembler is that not all operating system functions can be accessed in high level languages. The application program interfaces of IBM's mainframe operating systems is defined as a set of assembly language "macro" instructions, that typically invokeSupervisor Call (SVC) [e.g., on z/OS] or Diagnose (DIAG) [on, e.g., z/VM] instructions to invoke operating system routines. It is possible to use operating system services from programs written in high-level languages by use of assembler subroutines.
The format of assembler language statements reflects the layout of an 80-column punched card, though successive versions have relaxed most of the restrictions.
Basic Assembly language also permits an alternate statement format with the statement starting in column 25, allowing the assembled instruction to be punched into the same card beginning in column 1. This option was not continued in later versions of the assembler.
Three main types of instructions are found in the source code of a program written in assembler.
Assembler instructions, sometimes termeddirectives, 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 isUSING, 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 adisplacement (0–4095 bytes) from the value in abase 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 relatedDROP assembler instruction nullifies a previousUSING.
There is a one-to-one relationship withmachine instructions. The full mnemonic instruction set is described in thePrinciples 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 asX86 assembly language, register mnemonics are not reserved symbols but are defined throughEQU 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 notableinstruction mnemonics areBALR[b] for a call storing the return address and condition code in a register,SVC,[c]DIAG,[d] andZAP.[5]
System/360 machine instructions are one, two, or threehalfwords 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 supportmacros. 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 asAIF (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 theoperating system, and the OS provides standard macros for requesting those services. These are analogous toUnixsystem calls. For instance, inMVS (later z/OS),STORAGE (with theOBTAIN parameter) dynamically allocates a block of memory, andGET 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 following fragment shows how the logic "If SEX = 'M', add 1 to MALES; else, add 1 to FEMALES" would be performed in assembler.
CLI SEX,C'M' Male? BNE IS_FEM If not, branch around L 7,MALES Load current value of MALES into register 7 LA 7,1(7) add 1 ST 7,MALES store back the result B GO_ON Finished with this portion IS_FEM EQU * A label L 7,FEMALES Load current value in FEMALES into register 7 LA 7,1(7) add 1 ST 7,FEMALES store back the result GO_ON EQU * - rest of program - * MALES DC F'0' Counter for MALES (initially=0) FEMALES DC F'0' Counter for FEMALES (initially=0)
The following is the ubiquitous"Hello, World!" program, and would, executing under an IBM operating system such asOS/VS1 orMVS, display the words 'Hello, World!' on the operator's console:
HELLO CSECT The name of this program is 'HELLO' * Register 15 points here on entry from OPSYS or caller. STM 14,12,12(13) Save registers 14,15, and 0 thru 12 in caller's Save area LR 12,15 Set up base register with program's entry point address USING HELLO,12 Tell assembler which register we are using for pgm. base LA 15,SAVE Now Point at our own save area ST 15,8(13) Set forward chain ST 13,4(15) Set back chain LR 13,15 Set R13 to address of new save area * -end of housekeeping (similar for most programs) - WTO 'Hello, World!' Write To Operator (Operating System macro) * L 13,4(13) restore address to caller-provided save area XC 8(4,13),8(13) Clear forward chain LM 14,12,12(13) Restore registers as on entry DROP 12 The opposite of 'USING' SR 15,15 Set register 15 to 0 so that the return code (R15) is Zero BR 14 Return to caller * SAVE DS 18F Define 18 fullwords to save calling program registers END HELLO This is the end of the program
WTO is an assembler macro that generates an operating system call. Because of saving registers and later restoring and returning, this small program is usable as a batch program invoked directly by the operating systemJob control language (JCL) like this:
//EXECPGM=HELLO
or, alternatively, it can be CALLed as a subroutine from such a program:
CALL 'HELLO'
With the exception of the assemblers for theIBM System/360 Model 20, the IBM assemblers were largely upward-compatible. The differences were mainly in the complexity of expressions allowed and in macro processing.OS/360 assemblers were originally designated according to their memory requirements.
The assembler forBPS 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 orextended 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 allowdummy sections (structure definitions). Parenthesized expressions are not allowed and expressions are limited to three terms with the only operators being '+', '-', and '*'.[7]: 59–61
TheBasic 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 theDOS/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 atsystem generation (SYSGEN).
Assembler H runs onOS/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 theAMODE andRMODE 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 orHLASM 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 isIBM's currentassembler programming language for itsz/OS,z/VSE,z/VM andz/TPFoperating systems onz/Architecturemainframecomputers. Release 6 and later also run onLinux, and generateELF orGOFF object files (this environment is sometimes referred to asLinux on IBM Z).[19] While working at IBM, John Robert Ehrman created and was the lead developer for HLASM[e] 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 ahigh-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 theSLAC (Stanford Linear Accelerator) modifications. Among features added were an indication ofCSECT/DSECT for location counter, dependent[f] and labelled[g]USING statements, a list ofUSING statements currently active, an indication of whether a variable is read or written in the cross-reference, and allowing mixed-case symbol names.[18]TheRSECT 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."[22]: 41
TheHigh Level Assembler Toolkit is a separately priced accompaniment to the High Level Assembler. The toolkit contains:[23]
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 a7090 or 7094 system and was used while System/360 was in development.[7][24] This assembler supports six-bitBCD character set as well as eight-bitEBCDIC.
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 instructionsCIO,TIO,XIOB,SPSW,BAS,BASR, andHPR.[25]: 110 The Basic Assembler is a slightly more restricted version of System/360 Basic Assembler;[26] 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 toIOCS macros. The card versions are two-pass assemblers that only support card input/output. The tape-resident versions are one-pass, usingmagnetic tape for intermediate storage. Programs assembled with the CPS Assembler can address a maximum of 16 KB.[26]: 7–8
The DPS/TPS assembler is a somewhat restricted version of System/360 BPS/BOS Assembler.[25]: 132–134
TheIBM 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 acard image source dataset, named common, and implicit definition ofSETA assembler variables.[27]
It has no support for storage-to-storage (SS) instructions or theconvert to binary (CVB),convert to decimal (CVD),read direct (RDD) andwrite direct (WRD) instructions.[28] It does include four instructions unique to the Model 44:Change Priority Mask (CHPM),Load PSW Special (LPSX),Read Direct Word (RDDW), andWrite 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 andENDUP).[28]: 53, 73
The assembler for theSystem/360 Model 67 Time Sharing System has a number of differences in directives to support unique TSS features. ThePSECT directive generates aPrototype Control Section containing relocatable address constants and modifiable data used by the program.[29]: 143
"Assembler G" is a set of modifications made to Assembler F in the 1970s by theUniversity 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.[30] "Assembler G" was never an IBM product.
There have been several IBM-compatible assemblers for special environments.[31]
Originally all System/360 operating systems were written in assembler language, and all system interfaces were defined by macro definitions. Access from high-level languages (HLLs) was restricted to what that language supplied, and other system calls had to be coded as assembler subroutines called from HLL programs. Also, IBM allowed customization of OS features by an installation through what were known asExits—user-supplied routines that could extend or alter normal OS functions. These exits were required to be coded in assembler language. Later, IBM recoded OS/360 in a systems programming language,PL/S, but, except for a short trial, decided not to release the PL/S compiler to users. As a result of these factors, assembler language saw significant use on IBM systems for many years.
BASR and similar instructions.SVC have been replaced by aPC instruction.DIAG as anHVC instruction.USING is one that specifies a relocatable expression instead of a list of registers:USING IHADCB,SYSPRINT ... TM DCBOPTCD,DCBOPTC Test OPTCD in SYSPRINT ...
USING is one that only affects instructions that explicitly refer to it by qualifying an expression with a label:LA R4,SYSIN LA R5,SYSPRINTIN USING IHADCB,R4OUT USING IHADCB,R5 ... TM IN.DCBOFLGS,DCBOFTM Test OFLGS in SYSIN ... TM OUT.DCBOPTCD,DCBOPTC Test OPTCD in SYSPRINT ...
