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CS/COE0447 Computer Organization & Assembly Language

CS/COE0447 Computer Organization & Assembly Language. Chapter 2 Part 4. Topics. Creating executables Assembler, linker, loader Compilers versus interpreters What does the assembler do for you? Review of branch and jump instructions. void swap(int v[], int k) { int temp; temp = v[k];

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CS/COE0447 Computer Organization & Assembly Language

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  1. CS/COE0447Computer Organization & Assembly Language Chapter 2 Part 4

  2. Topics • Creating executables • Assembler, linker, loader • Compilers versus interpreters • What does the assembler do for you? • Review of branch and jump instructions

  3. void swap(int v[], int k) { int temp; temp = v[k]; v[k] = v[k+1]; v[k+1] = temp; } swap: muli $2, $5, 4 add $2, $4, $2 lw $15, 0($2) lw $16, 4($2) sw $16, 0($2) sw $15, 4($2) jr $31 00000000101000010… 00000000000110000… 10001100011000100… 10001100111100100… 10101100111100100… 10101100011000100… 00000011111000000… assembler compiler “C Program” Down to “Numbers”

  4. To Produce an Executable source file .asm/.s object file .obj/.o assembler source file .asm/.s object file .obj/.o executable .exe assembler linker source file .asm/.s object file .obj/.o library .lib/.a assembler

  5. Linker

  6. An Object File • Header • Size and position of other pieces of the file • Text segment • Machine codes • Data segment • Binary representation of the data in the source • Relocation information • Identifies instructions and data words that depend on absolute addresses • Symbol table • Keeps addresses of global labels • Lists unresolved references • Debugging information • Contains a concise description of the way in which the program was compiled

  7. An Assembler • Expands macros • Macro is a sequence of operations conveniently defined by a user • A single macro can expand to many instructions • Determines addresses and translates source into binary numbers • Record in “symbol table” addresses of labels • Resolve branch targets and complete branch instructions’ encoding • Record instructions that need be fixed after linkage • Packs everything in an object file • “Two-pass assembler” • To handle forward references • bne $t0,$t1,next_label • next_label:

  8. Assembler Directives • Guides the assembler to properly handle following codes with certain considerations • .text • Tells assembler that codes follow • .data • Tells assembler that data follow • .align • Directs aligning the following items • .global • Tells to treat the following symbol as global • .asciiz • Tells to handle the following as a “string”

  9. .data int_str: .asciiz “%d” .text .macro print_int($arg) la $a0, int_str mov $a1, $arg jal printf .end_macro … print_int($7) la $a0, int_str mov $a1, $7 jal printf Macro Example

  10. Branch and Jump Instructions Review

  11. Instruction Format • beq, bne PC = PC + 4 + BranchAddr • BranchAddr = {14{immediate[15]},immediate,2'b0} • Two issues: • Assembly  machine code • Execution of the machine code I op rs rt 16-bit immediate

  12. 0x00400024 0x00400028 0x0040002c 0x00400030 bne $t0,$s5,exitloop addi $s3,$s3,1 j loop exitloop: add $s6,$s3,$zero BNE machine code in binary: 000101 01000 10101 0000000000000010 BNE machine code in hex: 15150002 BranchAddr = {14{immediate[15]},immediate,2'b0} = {00000000000000,0000000000000010,00} = 0000 0000 0000 00 00 0000 0000 0000 1000 = 0x00000008 When BNE instruction is executed (condition true): Next address = PC + 4 + BranchAddr Next address = 00400024 + 4 + 00000008 = 00400030… address of the exitloop inst!

  13. BranchAddr: Why 2’b0 at the end? • BranchAddr might have been: • {number,00} = number * 4 (like shifting left by 2) • Recall: the immediate field of the instruction contains the number of instructions away the label is • Each instruction is 4 bytes, so multiplying by 4 gives you the number of bytes away it is. This is the number to add to PC + 4 to jump to the label. {16{immediate[15]},immediate}

  14. BranchAddress: Why 2’b0 at the end? • If immediate instead were the number of bytes away the label is, then we would be wasting 2 bits • Since all instruction addresses are multiples of 4, the bottom 2 bits are always 00 • By not including those bits in the immediate field of the machine code, branch instructions can be used to jump 4 times further away

  15. Instruction Format, cont’d • The address of next instruction is obtained by concatenating with PC PC = {PC[31:28],address,2’b0} Jump op 26-bit address

  16. 0x00400018 bne $s4, $s5, ELSE 0x0040001c add $s3, $s2, $s5 0x00400020 j EXIT ELSE: 0x00400024 sub $s3, $s2, $s5 0x00400028 addi $s5, $s5, 1 0x0040002c EXIT: addi $s4,$s4,1 j instruction machine code: Hex: 0810000b Look at execution: • PC = {PC[31:28],address,00} • PC[31:28] = 0000 • address = 00 0001 0000 0000 0000 0000 1011 • {0000, address, 00} = • 0000 00 0001 0000 0000 0000 0000 1011 00 BIN • 0 0 4 0 0 0 2 c HEX • The address EXIT stands for!

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