title: From-Machine-Language-to-Assembly-Language
date: 2023-11-20
author:
- AllenYGY
status: DONE
tags:
- NOTE
- CO
- Lec9
created: 2024-01-16T21:03
updated: 2024-05-31T01:16
publish: True
From-Machine-Language-to-Assembly-Language
Assembler is a program that turns symbols into binary machine instructions.
Assembly language Syntax
- Each line of a program is one of the following:
- an instruction
- an assembler directive (or pseudo-op)
- a comment
- Whitespace (between symbols) and case are ignored.
- Comments (beginning with “;”) are also ign
Assembly Language Instruction
LABEL OPCODE OPERANDS COMMENTS
- optional: Label, Comments
- mandatory: Opcode, Operands
Opcode and Operands
- Opcodes:
- reserved symbols that correspond to LC-3 instructions
- example: ADD, AND, LD, LDR
- Operands：
- Registers:
- Numbers: #(DEC) or x(HEX)
- Label: symbolic name oof memory location
Labels
placed at the beginning of the line
assigns a symbolic name to the address corresponding to line
Loop ADD R1,R1,#-1
Brp LOOP
- Anything after a semincolon is comments
- Ignored by assembler
- Used by humans to document/ understand programs
- Tips for useful comments:
- avoid restating the obvious, as “decrement R1”
- provide additional insight, as in “accumulate product in R6”
- use comments to separate pieces of program
Assembler Directives
- Do not refer to operations executed by program
- used by assembler
- look like instruction, but "opcode" starts whith dots
| Opcode |
Operand |
Meaning |
| .ORIG |
address |
starting address of program |
| .END |
end of program |
|
| .BLKW |
n |
allocate n words of storage |
| .FILL |
n |
allocate one word,initializa with value n |
| .STRINGZ |
n-character string |
allocate n+1 locations, initialize w/characters and null terminator |