Chapter 7Chapter 7LC-3 Assembly Language2Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyCS RealityCS Reality You’ve got to know assembly Chances are, you’ll never write program in assembly– Compilers are much better & more patient than you are Understanding assembly key to machine-level execution model– Behavior of programs in presence of bugs High-level language model breaks down– Tuning program performance Understanding sources of program inefficiency– Implementing system software Compiler has machine code as target Operating systems must manage process state3Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyIt’s hard to write code in 1’s & 0’s!It’s hard to write code in 1’s & 0’s! Assembly language makes it possible to write Machine Language code– each line of assembly language is translated into a single ML instruction A program called the Assembler does the translation and provides useful tools:– use of labels - symbolic names for address locations– automatic conversion of binary / hex / decimal– pseudo-ops4Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyAssembly Language InstructionsAssembly Language Instructions Formats– LABEL OPCODE OPERANDS ; COMMENTS– LABEL PSEUDO-OPS ; COMMENTS Opcode– Symbolic name for the 4-bit ML opcode Label– Symbolic name for a memory location. It is used to: indicate the target of a branch instruction, e.g. AGAIN in location 0B indicate the location of a stored value or array, e.g. NUMBER and SIX Comments– intended for humans only: explanation of code, visual display5Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyPseudoPseudo--Ops … Ops … – … are directives to the assembler they are not translated into ML instructions– LC-3 Pseudo-Ops: .ORIG address Tells assembler where to locate the program in memory (starting address). .FILL value Store value in the next location in memory. .BLKW n Set aside a block of n words in memory. .STRINGZ string Store the string, one character per word, in memory. Add a word of x0000 after the string. .END Marks the end of the source program (not to be confused with the instruction HALT!) .EXTERNAL The label so indicated is allocated in another module.6Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyA sample programA sample program01 ;02 ; Program to multiply an integer by the number 603 ;04 .ORIG x305005 LD R1, SIX06 LD R2, NUMBER07 AND R3, R3, #0 ; clear R3 to hold the product08 ;09 ; The inner loop0A ;0B AGAIN ADD R3, R3, R20C ADD R1, R1, #-1 ; keep track of iterations0D BRp AGAIN0E ;0F HALT10 ;11 NUMBER .BLKW 112 SIX .FILL x000613 ;14 .END7Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyFrom Assembly to bits…From Assembly to bits….ORIG x3000AND R1, R1, #0ADD R1, R1, #10LD R2, TwentyLD R3, EssHALTTwenty FILL x0014.BLKW 2Ess .FILL “S” .STRINGZ “Hi”.BLKW 3.ENDx3000: AND R1, R1, 0 0000x3001: ADD R1, R1, 0 1010x3002: LD R2, 0 0000 0010x3003: LD R3, 0 0000 0100x3004: TRAP 0010 0101 x3005: 0000 0000 0001 0100 ; x0014x3006:x3007:x3008: 0000 0000 0101 0011 ; x0053 = ‘S’x3009: 0000 0000 0100 1000 ; x0048 = ‘H’x300A: 0000 0000 0110 1001 ; x0069 = ‘i’x300B: 0000 0000 0000 0000 ; x0000 = null terminatorx300C:x300D:x300E:8Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyThe Assembly ProcessThe 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 assembly9Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyTwoTwo--Pass Assembly Pass Assembly -- 11First Pass - generating the symbol table–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 .END is encountered10Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblySymbol Table example Symbol Table example – Using the earlier example:Symbol AddressAgain x3053Number x3057Six x3058;; Program to multiply a number by six;.ORIG x3050x3050 LD R1, SIXx3051 LD R2, NUMBERx3052 AND R3, R3, #0;; The inner loop;x3053 AGAIN ADD R3, R3, R2x3054 ADD R1, R1, #-1 x3055 BRp AGAIN;x3056 HALT;x3057 NUMBER .BLKW 1x3058 SIX .FILL x0006;.END11Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyTwoTwo--Pass Assembly Pass Assembly -- 22Second Pass - generating the ML program–Scan each line again–Translate each AL instruction into ML Look up symbols in the symbol table instruction Ensure that labels are no more than +256 / -255 lines from instruction Determine operand field for the instruction–Fill memory locations as directed by pseudo-ops–Stop when .END is encountered12Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyAssembled codeAssembled code– Using the earlier example:Symbol AddressAgain x3053Number x3057Six x3058x3050 0010 001 0 0000 0111 ; LD R1, SIXx3051 0010 010 0 0000 0101 ; LD R2, NUMBERx3052 0101 011 011 1 00000 ; AND R3, R3, #0x3053 0001 011 011 0 00 010 ; ADD R3, R3, R2x3054 0001 001 001 1 11111 ; ADD R1, R1, #-1 x3055 0000 001 1 1111 1101 ; BRp AGAINx3056 1111 0000 0010 0101 ; HALTX3057 ; .BLKW 1x3058 0000 0000 0000 0110 ; .FILL x000613Wright State University, College of EngineeringDr. Doom, Computer Science & EngineeringCEG 320/520Comp. Org. & AssemblyAnother exampleAnother example.ORIG x30003000 AND