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More on Assembler

Human-Readable Machine Language

Computers like 1s and 0s:

0001001001100001

People like 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 codeLD R0, FIRSTLD R1, SECONDNOT R1, R1ADD R1, R1, #1 ADD R2, R0, R1ST R2, DIFFHALT

; Data variablesFIRST .BLKW 1SECOND .BLKW 1DIFF .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, FIRSTLD R1, SECONDNOT R1, R1ADD R1, R1, #1 ADD R2, R0, R1ST R2, DIFFHALTFIRST .BLKW 1SECOND .BLKW 1DIFF .BLKW 1

// Symbol table// Scope level 0:// Symbol Name Page Address// ---------------- ------------// FIRST 3007// SECOND 3008// DIFF 3009

Second Pass: Generate Machine CodeFor 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 argumentsex: NOT R1,#7

ADD R1,R2ADD R3,R3,NUMBER

Immediate argument too largeex: ADD R1,R2,#1023

Address (associated with label) more than 256 from instruction; can’t 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 imageinto memory

• more sophisticated loaders are able to relocate imagesto fit into available memory

• must readjust branch targets, load/store addresses

The Linker

Linking is the process of resolving symbols betweenindependent 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