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Introduction to Computing Systems from bits & gates to C & beyond Chapter 7 LC-2 Assembly Language Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside 7-2 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 Example ; ; Program to multiply a number by six ; .ORIG x3050 LD R1, SIX LD R2, NUMBER AND R3, R3, #0 ; clear R3 ; ; The inner loop ; AGAIN ADD R3, R3, R2 ADD R1, R1, #-1 ; keep track of iterations BRp AGAIN ; HALT ; NUMBER .BLKW 1 SIX .FILL x0006 ; .END Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside Assembly Language Instructions Formats  LABEL  LABEL OPCODE OPERANDS PSEUDO-OPS ; COMMENTS ; COMMENTS Label  Label is a symbolic name that refers to a memory location.  It is used to:  indicate the target of a branch instruction, e.g. AGAIN in location 0B  store a value in a memory location, e.g. NUMBER and SIX Comments  intended for humans only: explanation of code, visual display 7-3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside Pseudo-Ops  Are directives to the assembler  Assembler: the program that will translate human-readable assembly language into machine instructions.  LC-2 Pseudo-Ops:  .ORIG address Tells assembler where to locate the program in memory (starting address).  .END Marks the end of the source program.  .BLKW n Set aside a block of n words in memory.  .FILL value Store value in the next location in memory  .STRINGZ string Store the string, one character per word, in memory. Add a word of x0000 after the string.  .EXTERNAL 7-4 The label so indicated is allocated in another module. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside Examples Twenty Ess 7-5 .ORIG x3000 AND R1, R1, #0 ADD R1, R1, #10 LD R2, Twenty LD R3, Ess FILL x0014 .FILL “S” .BLKW 2 .STRINGZ “Hi” .BLKW 3 .END x3000: x3001: x3002: x3003: x3004: x3005: x3006: x3007: x3008: x3009: x300A: x300B: x300C: x300D: AND R1, R1, #0 ADD R1, R1, #10 LD R2, #4 LD R3, #5 x0014 ; 0000 0000 0001 0100 x0053 ; 0000 0000 0101 0011 x0048 ; H x0069 ; i x0000 ; null terminated string Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside The Assembly Process Objective  Translate the AL (Assembly Language) program into ML (Machine Language).  Each AL instruction yields one ML instruction word.  Interpret pseudo-ops correctly. Problem  An instruction may reference a label.  If the label hasn’t been encountered yet, the assembler can't form the instruction word Solution  Two-pass assembly 7-6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside Two-Pass Assembly - 1 First Pass  Scan each line  Keep track of current address  Increment by 1 for each instruction  Adjust as required for any pseudo-ops (e.g. .FILL or .STRINGZ, etc.)  For each label  Enter it into the symbol table  Allocate to it the current address  Stop when find .END 7-7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside Two-Pass Assembly - 2 Second Pass  Scan each line again  Translate each AL instruction into ML  Look up any symbols in the symbol table  Determine operand field for the instruction  Check for errors if reference crosses page boundaries  A reference to an undefined symbol is an error  Fill memory locations as directed by pseudo-ops  Stop when find .END 7-8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside Symbol ;Table  From the previous example: Symbol Address Again x3053 Number x3057 Six x3058 x3050 x3051 x3052 x3053 x3054 x3055 x3056 x3057 x3058 7-9 ; Program to multiply a number by six ; .ORIG x3050 LD R1, SIX LD R2, NUMBER AND R3, R3, #0 ; ; The inner loop ; AGAIN ADD R3, R3, R2 ADD R1, R1, #-1 BRp AGAIN ; HALT ; NUMBER .BLKW 1 SIX .FILL x0006 ; .END Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside Object File Each source file is translated into an object file: the executable image.  A complete program may include several source and/or object files:   Source files written by the programmer  Library files provided by the system (OS or other)  The object files must be linked  Need to provide a copy of the symbol table  Include it in the header of the object file 7 - 10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slides prepared by Walid A. Najjar & Brian J. Linard, University of California, Riverside Example - Parity Parity is a function that returns a 1 when the number of 1s in a word is odd and 0 when it is even. Symbol Address Count x3002 Shift x3005 Report Input x3007 x300A Output x300B 7 - 11 3000 3001 3002 3003 3004 3005 3006 3007 3008 3009 300A 300B .ORIG AND LDI Count BRz BRp ADD Shift ADD BRnzp Report AND STI TRAP Input .FILL Output .FILL x3000 R2, R2, x0 R1, Input Report Shift R2, R2, x1 R1, R1, R1 Count R3, R2, x1 R3, Output x25 x3200 x3201 ; clear R2 ; load word into R1 ; if 0, done counting ; if >0, skip ADD ; increment count ; shift left 1 bit ; go back up ; LSB 1 or 0? ; store results ; halt program ; address of input ; address of output