IBM Basic Assembly Language and successors


Basic Assembly Language is the commonly used term for a low-level programming language used on IBM System/360 and successor mainframes. Originally, "Basic Assembly Language" applied only to an extremely restricted dialect designed to run under control of IBM Basic Programming Support on systems with only 8 KB of main memory, and only a card reader, a card punch, and a printer for input/output — thus the word "Basic". However, the full name and the initialism "BAL" almost immediately attached themselves in popular use to all assembly-language dialects on the System/360 and its descendants. BAL for BPS/360 was introduced with the System/360 in 1964.
Assemblers on other System/360 operating systems through System/370, System/390, and System z, as well as the UNIVAC Series 90 mainframes made by Sperry Corporation, and the BS2000 Mainframes currently made by Fujitsu, inherited and extended its syntax. The latest derived language is known as the IBM High-Level Assembler. Programmers utilizing this family of assemblers also refer to them as ALC,, or simply "assembler".
BAL is also the mnemonic of the "Branch And Link".

General characteristics

The architecture of IBM mainframes has undergone several extensions over the years, including System/360, System/370, System/370 XA, ESA/370, ESA/390, and z/Architecture. Each of these architectures has retained compatibility with most of the features of its predecessor. BAL uses the native instruction set of these machines. It is thus closer to the hardware than third-generation languages such as COBOL.
The instruction set consists of the low-level operations supported by the hardware, such as:
InstructionMnemonicDescription
LOADLcopy a value from memory to a register
STORESTcopy a value from a register to memory
COMPARECcompare a register value with a value in memory
ShiftSLL, SRLmove the bits of a register left or right
START SUBCHANNELSSCHstart a sub-channel I/O operation using a string of Channel Command Words

The extreme simplicity of these operations means that the source code of a program written in assembler will usually be much longer than an equivalent program in, say, COBOL or Fortran. 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.
Another reason to use assembler is that not all operating system functions can be accessed in high level languages. The application program interface of mainframe operating systems is defined as a set of assembly language "macro" instructions, that typically invoke Supervisor Call or Diagnose hardware 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.

Assembler Statement Format

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.

Types of instructions

Three main types of instructions are found in the source code of a program written in assembler.

Machine instructions (mnemonic)

There is a one-to-one relationship with machine instructions. The full mnemonic instruction set is described in the Principles of Operation manual for each processor.
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'
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 mnemonics are BALR for a call storing the return address in a register before stacks were used, SVC, DIAG, and ZAP. The latter inspired the name of the SuperZAP utility by a programmer using the pseudonym WAMOZART, cf..
System/360 machine instructions are one, two, or three halfwords in length. Originally there were four instruction formats, designated by the first two bits of the operation code field; z/Architecture added additional formats.

Assembler instructions

Assembler instructions, sometimes termed directives 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"; DC defines a constant to be placed in the object code.

Macros and conditional assembly

Basic assembler language does not support macros.
Later assembler versions 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, 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.

Operating system macros

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, STORAGE dynamically allocates a block of memory, and GET retrieves the next logical record from a file.
Unlike Unix system calls, macros are not standardized across operating systems though. Even something as simple as writing a "sequential file" is coded differently e.g. in Z/OS than in Z/VSE.

Examples

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
AL 7,=F'1' 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
AL 7,=F'1' add 1
ST 7,FEMALES store back the result
GO_ON EQU * - rest of program -
*
MALES DC F'0' Counter for MALES
FEMALES DC F'0' Counter for FEMALES

The following is the ubiquitous Hello world program, and would, executing under an IBM operating system such as OS/VS1 or MVS, 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 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 Set forward chain
ST 13,4 Set back chain
LR 13,15 Set R13 to address of new save area
* -end of housekeeping -
WTO 'Hello World' Write To Operator
*
L 13,4 restore address to caller-provided save area
XC 8,8 Clear forward chain
LM 14,12,12 Restore registers as on entry
DROP 12 The opposite of 'USING'
SR 15,15 Set register 15 to 0 so that the return code 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 system Job control language like this:

// EXEC PGM=HELLO

or, alternatively, it can be CALLed as a subroutine from such a program:

CALL 'HELLO'

Versions

With the exception of the assemblers for the IBM 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.

7090/7094 Support Package assembler

This cross-assembler runs on a 7090 or 7094 system and was used while System/360 was in development.

Basic Programming Support assembler

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. It has no support for macro instructions or extended mnemonics. It can assemble only a single control section and does not allow dummy sections. Parenthesized expressions are not allowed and expressions are limited to three terms with the only operators being '+', '-', and '*'.

Basic Operating System assembler

The Basic Operating System has two assembler versions. Both require 16 KB memory, one is tape resident and the other disk.

Assembler D

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.

Assembler E and F

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. 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. These assemblers are a standard part of OS/360; the version that was generated was specified at system generation.

Model 44 Programming System Assembler

"With certain exceptions, the IBM System/360 Model 44 Programming System Assembler Language is a selected subset of the languages available in the IBM System/360 programming support." Most significantly the Model 44 assembler lacked support for macros and continuation statements. On the other hand it had 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.

Assembler G

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

Assembler H

Assembler H Version 2 was announced in 1981 and includes support for Extended Architecture, including the AMODE and RMODE directives. It was withdrawn from marketing in 1994 and support ended in 1995. It was replaced by High Level Assembler.

Assembler XF

Assembler XF is an upgrade of Assembler F which includes the new System/370 architecture instructions. This version provides a common assembler for OS/VS and DOS/VS systems. Other changes include relaxing restrictions on expressions and macro processing. Assembler XF requires a minimum partition/region size of 64 KB. Recommended size is 128 KB.

High Level Assembler

High Level Assembler, announced in 1992 as a licensed program "becomes the default translator for System/370 and System/390 operating environments." The assembler supports the MVS, VSE, and VM operating systems and successors. As of Release 6 it now is able to run under Linux on IBM Z and generate ELF or COFF object files. It features a long list of mostly usability enhancements, and incorporates the SLAC modifications to Assembler H. Among features added were an indication of CSECT/DSECT for location counter, a list of "using" registers currently active, an indication of whether a variable is read or written in the cross-reference, and allowing mixed-case symbol names. The RSECT directive allows the assembler to check reentrancy on a per-section basis. RSECT was previously "undocumented and inconsistently implemented in Assembler H." HLASM is the current assembler for IBM mainframe systems as of 2012.

Specialized versions

IBM System/360 Model 44 PS assembler

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. It has no support for storage-to-storage instructions or the convert to binary, convert to decimal, read direct and write direct instructions. It does include four instructions unique to the Model 44: Change Priority Mask, Load PSW Special, Read Direct Word, and Write Direct Word. It also includes directives to update the source program, a function performed by utility programs in other systems. It provides named common and implicitly defined &SETA symbols, but has some restrictions as well.

IBM System/360 TSS assembler

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.

Non-IBM assemblers

There have been several IBM-compatible assemblers for special environments.