Assembler

From C64-Wiki

Assembler is a language which translates a CPUs native code in a human readable form. Not only the language but also the compiler is called Assembler. Turning the assembly source code into machine language is called "assemble". The reverse procedure is called "disassemble" which turns machine code in readable code.

1 General
2 Advantages / Disadvantages
- 2.1 Advantages
- 2.2 Disadvantages
3 Examples
4 Critique
5 Links & Literature

[edit] General

Every processor architecture (e.g. x86, risc) has its own functions and thereby differing assembler commands. Still most of them show similarities. Machine commands are built from numbers which are stored as Binary code in the computers memory. To make it possible to program with this commands each opcode has a short symbol called Mnemonics.

A assembler (compiler) translates these Mnemonics into their corresponding opcodes and data. An example:

*=$c000

lda #$00      ; load the number 0 into the accumulator register
sta $d020     ; store the content of the accumulator in the register for the border color
...

... inside the RAM of the C64 this is represented like this:

Address  Opcode + Operator(s)

c000     A9 00           ; "C000" is the memory address, "A9" the Opcode, "00" the Operator
c002     8D 20 D0        ; "C002" is the memory address, "8D" the Opcode, "20" and "D0" are Operators
...

... as Binary code from $c000

1010100100000000100011010010000011010000

Some assemblers for the C64 (e.g. TurboAssembler, ACME (crosscompiler)) translate not only the mnemonics but also provide macros to reduce the common workload.

[edit] Advantages / Disadvantages

[edit] Advantages

As close to the machine as possible
depending on the skill of the programmer extremely fast
assembly programs are usually rather small

[edit] Disadvantages

difficult to learn
high workload
not portable

Still assembler is worth learning. As byproduct you get to understand the inner workings of a computer. Assembler still has its uses today. Programming of drivers would be almost impossible without assembler.

[edit] Examples

Set screen colors to black ... (sys 49152)

       *=$c000 

       lda #$00
       sta $d020
       sta $d021
       rts

Play music (expecting player routine at $1000) ... (sys 49152)

       *=$c000 

     sei
     lda #<irq
     ldx #>irq
     sta $0314
     stx $0315
     cli
     lda #$00
     tay
     tax
     jsr $1000
     rts

irq  lda $d012
     cmp #100
     bne irq
     jsr $1003
     jmp $ea31