HLASM Mastery
The Complete IBM Mainframe Assembler Course
IBM High Level Assembler (HLASM) is the lowest-level language on z/OS mainframes — used in banking cores, payment systems, and operating system internals where every microsecond counts. This course takes you from zero to production-ready, covering instructions, macros, linkage conventions, z/OS system services, and 64-bit z/Architecture.
HLASM Foundations
5 modules
What is HLASM? IBM Mainframe Assembler Overview and Use Cases
History of IBM Mainframe Assembler, why HLASM is still used in banking and enterprise, HLASM vs COBOL, and who writes assembler code in 2026.
Read article →HLASM Environment: JCL, ASMA90, and Your First Assembly Program
Setting up your HLASM environment on z/OS, writing JCL to invoke ASMA90, understanding the assembly process, and running your first HLASM program.
Read article →HLASM Statement Format: Labels, Operation Codes, and Operands
The four fields of an HLASM statement (name, operation, operands, remarks), continuation lines, comment statements, and the CSECT/END structure.
Read article →HLASM Registers: General Purpose Registers, PSW, and Conventions
The 16 general-purpose registers, the Program Status Word (PSW), register 0 and 1 conventions, base register setup, and the standard register usage table.
Read article →HLASM Data Representation: EBCDIC, Binary, Hex, and Packed Decimal
How z/OS represents data: EBCDIC character encoding, binary integers, hexadecimal notation, packed decimal format, and zoned decimal format.
Read article →Core HLASM Instructions
5 modules
HLASM Fixed-Point Arithmetic: L, ST, A, S, M, D Instructions
Loading and storing registers with L/ST, addition and subtraction with A/S, multiplication with M/MH, division with D, and register-to-register variants.
Read article →HLASM Logical Operations: AND, OR, XOR, and Shift Instructions
Bitwise AND (N/NR), OR (O/OR), XOR (X/XR), SHIFT (SLL/SRL/SLA/SRA), and practical uses for bit masking, flag testing, and data packing.
Read article →HLASM Branching: BC, BCR, BXH, BXLE and the Condition Code
The condition code and its four states, the Branch on Condition instruction (BC/BCR), extended mnemonics (BE/BNE/BL/BH), BXH/BXLE loop control, and structured loops.
Read article →HLASM Character Operations: MVC, MVZ, MVN, MVI, and TR
Moving character data with MVC, moving zones and numerics with MVZ/MVN, moving immediate values with MVI, and translating data with the TR instruction.
Read article →HLASM Compare Instructions: CLC, CLI, CR, C and Condition Codes
Comparing character strings with CLC, comparing immediate bytes with CLI, comparing registers with CR, comparing to storage with C, and using compare results for branching.
Read article →Data Structures and Storage
5 modules
HLASM Data Definition: DC, DS, and Storage Alignment
Defining constants with DC (Define Constant) and storage with DS (Define Storage), type codes (C, X, B, F, H, D, P, Z), duplication factors, and boundary alignment.
Read article →HLASM Packed Decimal Arithmetic: AP, SP, MP, DP, CP, PACK, UNPK
Converting between zoned and packed decimal with PACK/UNPK, packed arithmetic with AP/SP/MP/DP, comparing packed fields with CP, and handling decimal overflow.
Read article →HLASM Addressing: Base-Displacement and Indexed Addressing
How z/Architecture addressing works, setting up base registers with BALR, base-displacement addressing, indexed addressing, and the USING and DROP directives.
Read article →HLASM Macros: Defining, Invoking, and Parameterising Macros
Writing macro definitions with MACRO/MEND, positional and keyword parameters, conditional assembly with AIF/ANOP/AGO, local and global set symbols, and the macro library.
Read article →HLASM DSECT: Mapping Data Structures with Dummy Sections
Using DSECT to define storage maps without allocating memory, mapping input records and control blocks, USING a DSECT with a base register, and practical record layouts.
Read article →System Programming
4 modules
HLASM and z/OS: SVCs, MVS Macros, and System Services
Calling z/OS services with SVC instructions, common MVS macros (WTO, GETMAIN, FREEMAIN, OPEN, GET, PUT, CLOSE), and writing a simple z/OS utility program.
Read article →HLASM Linkage Conventions: Standard Linkage and Save Areas
The standard IBM linkage convention, saving and restoring registers, the save area chain, BALR and BASR for branching and linking, and passing parameters between programs.
Read article →HLASM with COBOL: Calling Assembler Routines from COBOL
Writing HLASM subroutines callable from COBOL, matching COBOL CALL linkage conventions, accessing COBOL data areas from assembler, and common mixed-language patterns.
Read article →Debugging HLASM: Storage Dumps, IPCS, and ESTAE
Reading a z/OS storage dump, using IPCS (Interactive Problem Control System) to analyse abends, coding ESTAE for error recovery, and common HLASM abend codes (0C1, 0C4, 0C7).
Read article →Advanced HLASM and Career
3 modules
64-bit HLASM: z/Architecture Instructions and AMODE 64
64-bit general registers, 64-bit instructions (LG, STG, AG, SG, MLG, DLG), AMODE 31 vs AMODE 64, and migrating 31-bit assembler code to 64-bit z/Architecture.
Read article →HLASM Performance Optimisation Techniques
Why assembler outperforms higher-level languages, instruction timing, cache-friendly code, avoiding pipeline stalls, using EX for variable-length operations, and benchmarking techniques.
Read article →50 HLASM Interview Questions and Answers (2026)
Top 50 HLASM interview questions covering assembler fundamentals, registers, instructions, macros, linkage, z/OS system services, debugging, and mainframe career guidance.
Read article →Who Is This Course For?
Built for developers at every stage — from first-time mainframe learners to engineers targeting senior z/OS roles
Mainframe Newcomers
No assembler experience? Start with HLASM Foundations to understand registers, addressing, and the z/OS execution model before writing your first program.
COBOL Developers
Already know COBOL? HLASM gives you the ability to write performance-critical routines, understand what COBOL compiles to, and debug at the machine level.
Systems Programmers
Targeting z/OS systems programming roles? The System Programming phase covers SVCs, MVS macros, linkage conventions, and IPCS debugging — exactly what banks and enterprises need.
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