Introduction to Computer Engineering CS ECE 252 Spring 2010 Prof Guri Sohi Computer Sciences Department University of Wisconsin Madison Chapter 7 9 2 Assembly Language and Subroutines Copyright The McGraw Hill Companies Inc Permission required for reproduction or display Human Readable Machine Language Computers like ones and zeros 0001110010000110 Humans like symbols ADD R6 R2 R6 increment index reg 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 7 3 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display 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 4 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display 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 7 5 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display 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 7 6 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display 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 7 7 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display 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 of program END 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 7 8 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display 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 7 9 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display Style Guidelines Use the following style guidelines to improve the readability and understandability of your programs 1 Provide a program header with author s name date etc and purpose of program 2 Start labels opcode operands and comments in same column for each line Unless entire line is a comment 3 Use comments to explain what each register does 4 Give explanatory comment for most instructions 5 Use meaningful symbolic names Mixed upper and lower case for readability ASCIItoBinary InputRoutine SaveR1 6 Provide comments between program sections 7 Each line must fit on the page no wraparound or truncations Long statements split in aesthetically pleasing manner 7 10 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display Sample Program Count the occurrences of a character in a file Remember this Count 0 R2 0 Done YES R1 EOT Ptr 1st file character Convert count to ASCII character R0 x30 R0 R2 R0 NO R3 M x3012 Print count YES Match NO TRAP x21 R1 R0 Input char from keybd TRAP x23 HALT Incr Count Load char from file TRAP x25 R2 R2 1 R1 M R3 Load next char from file R3 R3 1 R1 M R3 7 11 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display Char Count in Assembly Language 1 of 3 Program to count occurrences of a character in a file Character to be input from the keyboard Result to be displayed on the monitor Program only works if no more than 9 occurrences are found Initialization ORIG AND LD GETC LDR x3000 R2 R2 0 R3 PTR R1 R3 0 R2 R3 R0 R1 is counter initially 0 is pointer to characters gets character input gets first character Test character for end of file TEST ADD R4 R1 4 Test for EOT ASCII x04 BRz OUTPUT If done prepare the output 7 12 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display Char Count in Assembly Language 2 of 3 Test character for match If a match increment count NOT R1 R1 ADD R1 R1 R0 If match R1 xFFFF NOT R1 R1 If match R1 x0000 BRnp GETCHAR If no match do not increment ADD R2 R2 1 Get next character from file GETCHAR ADD R3 R3 1 Point to next character LDR R1 R3 0 R1 gets next char to test BRnzp TEST Output the count OUTPUT LD R0 ASCII Load the ASCII template ADD R0 R0 R2 Covert binary count to ASCII OUT ASCII code in R0 is displayed HALT Halt machine 7 13 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display Char Count in Assembly Language 3 of 3 Storage for pointer and ASCII template ASCII FILL x0030 PTR FILL x4000 END 7 14 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display 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