Computer Science 101 How the Assembler Works

Assembly Language Programming

Translation and Loading Assembler –Translates a symbolic assembly language program into machine language –Tasks performed Convert symbolic op codes to binary Convert symbolic addresses to binary Perform the assembler services requested by the pseudo-ops Put the translated instructions into a file for future use

Translation and Loading (continued) Op code table –Alphabetized list of all legal assembly language op codes and their binary equivalents In assembly language: –A symbol is defined when it appears in the label field of an instruction or data pseudo-op Pass –Process of examining and processing every assembly language instruction in the program, one instruction at a time

Structure of the Opcode Table

Translation and Loading (continued) First pass over source code – Assembler looks at every instruction Binding –Process of associating a symbolic name with a physical memory address Primary purposes of the first pass of an assembler –To bind all symbolic names to address values –To enter those bindings into a symbol table

Generating a Symbol Table

Generating a symbol table.begin in x load zero compare x jumpgt endif subtract x store x endif: output x halt x:.data 0 zero:.data 0.end

Translation and Loading (continued) Location counter –Variable used to determine the address of a given instruction Second pass –Assembler translates source program into machine language After completion of pass 1 and pass 2 –Object file contains the translated machine language object program

Generating machine code.begin in x load zero compare x jumpgt endif subtract x store x endif: out x halt x:.data 0 zero:.data 0.end load 0000 store 0001 clear 0010 add 0011 increment 0100 subtract 0101 decrement 0110 compare 0111 jump 1000 jumpgt 1001 jumpeq 1010 jumplt 1011 jumpneq 1100 in 1101 out 1110 halt 1111 Source program Opcode table