Computer Science 210 Computer Organization More on Assembler Human-Readable Machine Language Computers work with 1s and 0s: 0001001001100001 People work with symbols: ADD R1, R1, #1 ; Increment R1
The assembler makes this happen! Example: diff = first - second ;; Author: Ken Lambert ;; This program subtracts the number in the variable SECOND from FIRST ;; and stores the result in DIFF ;; Pseudocode design: ; diff = first - second .ORIG x3000 ;; Register usage: ; R0 = first ; R1 = second ; R2 = diff ; Program code
LD R0, FIRST LD R1, SECOND NOT R1, R1 ADD R1, R1, #1 ADD R2, R0, R1 ST R2, DIFF HALT ; Data variables FIRST .BLKW 1 SECOND .BLKW 1 DIFF .BLKW 1
.END The Assembly Process Convert the program in the source (.asm) file to an executable file (.obj) for the LC3 simulator First pass: Scan program file Find all labels and calculate their addresses, creating a symbol table Second pass: Convert instructions to machine language, using the symbol table First Pass: Construct the Symbol Table
1. Find the .ORIG statement, which tells us the address of the first instruction. Initialize location counter (LC), which keeps track of the current instruction. 2. For each non-empty line in the program: a) If line begins with label, add label and LC to symbol table. b) Increment LC. NOTE: If statement is .BLKW or .STRINGZ, increment LC by the number of words allocated. 3. Stop when .END statement is reached. NOTE: A line that contains only a comment is considered an empty line. Example Symbol Table
Code in subtract.asm Table in subtract.sym LD R0, FIRST LD R1, SECOND NOT R1, R1 ADD R1, R1, #1 // Symbol table // Scope level 0: // Symbol Name // ---------------// FIRST // SECOND
// DIFF ADD R2, R0, R1 ST R2, DIFF HALT FIRST .BLKW 1 SECOND .BLKW 1 DIFF .BLKW 1 Page Address -----------3007 3008
3009 Second Pass: Generate Machine Code For each executable assembly language statement, generate the corresponding machine language instruction If operand is a label, look up the address from the symbol table Potential errors to detect and flag: Improper number or type of arguments ex: NOT R1,#7 ADD ADD R1,R2
R3,R3,NUMBER Immediate argument too large ex: ADD R1,R2,#1023 Address (associated with label) more than 256 from instruction; cant use PC-relative addressing mode Object File Format An LC-3 object file contains Starting address (location where program must be loaded), followed by Machine language instructions Multiple Object Files
An object file is not necessarily a complete program. system-provided library routines code blocks written by multiple developers For LC-3 simulator, we can load multiple object files into memory, then start executing at a desired address. system routines, such as keyboard input, are loaded automatically loaded into system memory, below x3000 user code should be loaded between x3000 and xFDFF each object file includes a starting address be careful not to load overlapping object files In LC3, first file contains the program Remaining files contain data (run lc3convert b16 or b2) The Loader
Loading is the process of copying an executable image into memory more sophisticated loaders are able to relocate images to fit into available memory must readjust branch targets, load/store addresses The Linker Linking is the process of resolving symbols between independent object files suppose we define a symbol in one module, and want to use it in another some notation, such as .EXTERNAL, is used to tell assembler that a symbol is defined in another module linker will search symbol tables of other modules to resolve
symbols and complete code generation before loading For Friday Input and Output Operations Chapter 8
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