Introduction

A computer can understand and interpret only machine language. Machine language is in binary form and, thus, difficult to write. The assembler language is a symbolic programming language that you can use to code instructions instead of coding in machine language.

Because the assembler language lets you use meaningful symbols made up of alphabetic and numeric characters, instead of just the binary digits 0 and 1 used in machine language, you can make your coding easier to read, understand, and change. The assembler must translate the symbolic assembler language into machine language before the computer can run your program. The specific procedures to do this vary according to the system you are using. However, the method is basically the same for all systems:
Figure 1. Assembling and link-editing your assembler language program
  ┌───────────────────┐
  │ Your assembler    │
  │ language source   ├─────┐
  │ statements        │     │
  └───────────────────┘     │
                            V
                ┌───────────┴───────────┐
                │ HIGH LEVEL ASSEMBLER  │
                └─────────┬───┬─────────┘
                          │   │
                          │   │
  ┌──────────┐            │   │           ┌──────────────────┐
  │ Messages │            │   │           │ Machine language │
  │   and    ├<───────────┘   └──────────>┤ version of your  │
  │ listings │                            │ program          │
  │   .      │                            └─────────┬────────┘
  │   .      │                                      │
  └────┬─────┘                                      │
       ∧                                            │
       │                                            │
       │           ┌─────────────────┐              │
       │           │     LINKER      ├<─────────────┘
       │           └──────┬───┬──────┘
       │                  │   │
       │                  │   │           ┌───────────────────┐
       └──────────────────┘   └──────────>┤ Executable module │
                                          └───────────────────┘

Your program, written in the assembler language, is the source module that is input to the assembler. The assembler processes your source module and produces an object module in machine language (called object code). The object module is input to be processed by the linker or the binder. The linker or binder produces a load module (z/OS and CMS), or a phase (z/VSE), that the computer can load into its memory and run. Your source module and the object code produced are printed, along with other information, on a program listing.