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1 .Chapter 7 Assembly Language
2 .7- 2 Human-Readable Machine Language Computers like ones and zeros… Humans like symbols… Assembler is a program that turns symbols into machine instructions. ISA-specific: close correspondence between symbols and instruction set mnemonics for opcodes labels for memory locations additional operations for allocating storage and initializing data ADD R6,R2,R6 ; increment index reg. 0001110010000110
3 .Machine Language vs. Assembly Language Machine Language 0011 0000 0000 0000 0101 000 000 1 00000 0101 111 111 1 00000 0001 000 000 1 01100 0000 110 000000011 0001 111 111 000 100 0001 000 000 1 11111 0000 111 111111100 1111 0000 0010 0101 Assembly Language .ORIG x3000 AND R0, R0, #0 AND R7, R7, #0 ADD R0, R0 , #12 TEST BRnz DONE ADD R7, R7, R4 ADD R0, R0 , #- 1 BRnzp TEST DONE HALT .END 7- 3 Objective: Multiply the value stored in R4 by 12
4 .Machine Language vs. Assembly Language Machine Language 0011 0000 0000 0000 0101 000 000 1 00000 0101 111 111 1 00000 0001 000 000 1 ????? 0000 110 000000011 0001 111 111 000 100 0001 000 000 1 11111 0000 111 111111100 1111 0000 0010 0101 Assembly Language .ORIG x3000 AND R0, R0, #0 AND R7, R7, #0 ADD R0, R0 , ??? TEST BRnz DONE ADD R7, R7, R4 ADD R0, R0 , #- 1 BRnzp TEST DONE HALT .END 7- 4 Objective: Multiply the value stored in R4 by 120
5 .The Solution in Assembly Language . ORIG x3000 AND R0, R0, #0 AND R7, R7, # 0 LD R3, VAL ADD R0, R0, R3 TEST BRnz DONE ADD R7, R7, R4 ADD R0, R0, #-1 BRnzp TEST DONE HALT VAL .FILL #120 .END 7- 5 Objective: Multiply the value stored in R4 by 120
6 .7- 6 An Assembly Language Program ; ; Program to multiply a number by the constant 6 ; .ORIG x3050 LD R1, SIX LD R2, NUMBER AND R3, R3, #0 ; Clear R3. It will ; contain the product. ; The inner loop ; AGAIN ADD R3, R3, R2 ADD R1, R1, #-1 ; R1 keeps track of BRp AGAIN ; the iteration. ; HALT ; NUMBER .BLKW 1 SIX .FILL x0006 ; .END
7 .7- 7 Assembler Directives Pseudo-operations do not refer to operations executed by program used by assembler look like instruction, but “opcode” starts with dot 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, initialize with value n .STRINGZ n-character string allocate n+1 locations, initialize w/characters and null terminator
8 .7- 8 Trap Codes LC-3 assembler provides “pseudo-instructions” for each trap code, so you don’t have to remember them. Code Equivalent Description HALT TRAP x25 Halt execution and print message to console. IN TRAP x23 Print prompt on console, read (and echo) one character from keybd. Character stored in R0[7:0]. OUT TRAP x21 Write one character (in R0[7:0]) to console. GETC TRAP x20 Read one character from keyboard. Character stored in R0[7:0]. PUTS TRAP x22 Write null-terminated string to console. Address of string is in R0.
9 .7- 9 LC-3 Assembly Language Syntax Each line of a program is one of the following: an instruction an assember directive (or pseudo-op) a comment Whitespace (between symbols) and case are ignored. Comments (beginning with “;”) are also ignored. An instruction has the following format: LABEL OPCODE OPERANDS ; COMMENTS optional mandatory
10 .7- 10 Opcodes and Operands Opcodes reserved symbols that correspond to LC-3 instructions listed in Appendix A ex: ADD , AND , LD , LDR , … Operands registers -- specified by Rn, where n is the register number numbers -- indicated by # (decimal) or x (hex) label -- symbolic name of memory location separated by comma number, order, and type correspond to instruction format ex: ADD R1,R1,R3 ADD R1,R1,#3 LD R6,NUMBER BRz LOOP
11 .7- 11 Labels and Comments Label placed at the beginning of the line assigns a symbolic name to the address corresponding to line ex: LOOP ADD R1,R1,#-1 BRp LOOP Comment anything after a semicolon is a comment 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
12 .7- 12 Style Guidelines Use the following style guidelines to improve the readability and understandability of your programs: Provide a program header, with author’s name, date, etc., and purpose of program. Start labels, opcode, operands, and comments in same column for each line. (Unless entire line is a comment.) Use comments to explain what each register does. Give explanatory comment for most instructions. Use meaningful symbolic names. Mixed upper and lower case for readability. ASCIItoBinary, InputRoutine, SaveR1 Provide comments between program sections. Each line must fit on the page -- no wraparound or truncations. Long statements split in aesthetically pleasing manner.
13 .7- 13 Assembly Process Convert assembly language file (.asm) into an executable file (.obj) for the LC-3 simulator. First Pass: scan program file find all labels and calculate the corresponding addresses; this is called the symbol table Second Pass: convert instructions to machine language, using information from symbol table
14 .7- 14 First Pass: Constructing the Symbol Table Find the .ORIG statement, which tells us the address of the first instruction. Initialize location counter (LC), which keeps track of the current instruction. For each non-empty line in the program: If line contains a label, add label and LC to symbol table. Increment LC. NOTE: If statement is .BLKW or .STRINGZ , increment LC by the number of words allocated. Stop when .END statement is reached. NOTE: A line that contains only a comment is considered an empty line.
15 .7- 15 Second Pass: Generating Machine Language 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 problems: Improper number or type of arguments ex: NOT R1,#7 ADD R1,R2 ADD R3,R3,NUMBER Immediate argument too large ex: ADD R1,R2,#1023 Address (associated with label) more than 256 from instruction can’t use PC-relative addressing mode
16 .7- 16 LC-3 Assembler Using PennSim or CodeLup generates several different output files. This one gets loaded into the simulator.
17 .A few Practical Points Both PennSim and CodeLup include assembliers CodeLup accepts machine language (in text representation) but PennSim does not PennSim is not case sensitive CodeLup is somewhat case sensitive We will probably use PennSim as the assemblier for your projects Type as < fileName >. asm in the command line to assemble your code 7- 17
18 .7- 18 Object File Format LC-3 object file contains Starting address (location where program must be loaded), followed by… Machine instructions Example Beginning of “count character” object file looks like this: 0011000000000000 0101010010100000 0010011000010001 1111000000100011 . . . .ORIG x3000 AND R2, R2, #0 LD R3, PTR TRAP x23
19 .7- 19 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, 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
20 .7- 20 Linking and Loading 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 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