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Assembly Process Machine Code Generation Assembling a program entails translating the assembly language into binary machine code This requires more than simply mapping assembly instructions to machine instructions Each instruction is bound to an address Labels are bound to addresses Assembly instructions which refer to labels generate machine instructions which contain the label's address Pseudo-instructions are translated into one or more machine instructions Instruction Format addi $13,$7,50 0010 00 6 bits opcode 00111 01101 5 bits 5 bits 0000 0000 0011 0010 16 bits immediate operand add $13,$7,$8 0000 00 opcode 00 111 01000 01101 000 0010 0000 extended opcode The symbol table The assembler scans the source code and generates the appropriate bit string for each line encountered The assembler must remember what memory locations have been allocated to which address each label is bound A symbol table is a list of (label, address) pairs When the data and text segments have been generated, they are stored as an executable file The file is used by a program called the loader to initialize memory to the appropriate state before execution Instructions The .text directive tells the assembler that the lines which follow are instructions. By default, the text segment starts at 0x00400000 In some cases, a symbol may not have an assigned address yet when the assembler scans the line where it belongs A second pass through the code can update instructions containing unresolved labels Maintain a list of addresses in which each unresolved label appears When the labeled is added to the symbol table, all locations in the corresponding list are updated to hold the address associated with the label Branch offset in the MIPS R2000 In machine code, the target address in a branch must be specified as an offset from the address of the branch. During execution, this offset is simply added to the program counter to fetch the next instruction PC contains the address Offset is measured in words, not bytes PC_NEW = offset*4 + PC_OLD To calculate the offset, the assembler uses the formula: offset = (target instruction address – (branch instruction address))/4 Branch offset calculation The offset is stored in the instruction as a word offset rather than a byte offset. Instructions are only stored at word boundaries For both target and branch instruction, the least two bits of the address are zero An offset maybe negative If the target instruction preceded the branch instruction The offset is stored in the 16-bit immediate field This means the branch can only jump about 215 instructions before or after the current address 215 instructions (words) = 217 bytes Branch offset calculation An entry in the SPIM instruction list offset in bytes (__start = 0x00400000) 0x00400000 – (0x00400068) = - 104 stored offset ffe6 = -26 = -104/4 [0x00400068] 0x1440ffe6 offset calculation, in bytes ignores PC increment bne $2, $0, -104 [__start-0x00400068]; 44: bnez $v0, __start machine code orignal assembly code instruction address line number in source file Jump target calculation The jump instruction has two forms Pseudo-direct, for j and jal Register direct for jr and jalr jr and jalr specify a register containing the address to be loaded into the PC j and jal specify most of the address of the target within the instruction. However, they have a range of at most one-sixteenth of the memory space f e d c b a 9 8 7 6 5 4 3 2 1 0 Jump target calculation The target address is a 32 bit quantity Since all word addresses are multiples of 4 there is no need to store the last two bits The jump instruction format has 26 bits for the target address The remaining 6 bits of the instruction are used for the opcode The highest-order 4 bits of the target are taken from the address currently stored in the program counter PC opcode Jump target bits (26) 00 Jump Target Calculation jump instructions have a range of 226 words or 226 x 22 =228 bytes This range is NOT symmetric about the jump instruction +0x0fffff7c 0x80000080 -0x00000080 f e d c b a 9 8 7 6 5 4 3 2 1 0 Program relocation It is possible that program modules are developed separately by individual programmers. When these programs are to be loaded into memory they should not be assigned overlapping memory space. Thus,the modules have to be relocated relative addresses are relocatable Any absolute references must be "fixed" by the loader Use a logical base address known at load time Absolute addresses are stored as offsets from this TBD base From source to executable high-level source code lib obj asm exe asm obj linker compiler assembler loader memory Some examples of assembling code .data a1: .word 3 a2: .word 16, 16, 16, 16 a3: .word 5 .text __start: la $6, a2 loop: lw $7, 4($6) mul $9, $10, $7 b loop li $v0, 10 syscall Some examples of assembling code .data a1: .word 3 a2: .word 16, 16, 16, 16 a3: .word 5 .text __start: la $6, a2 loop: lw $7, 4($6) mult $9, $10, $7 b loop li $v0, 10 syscall Symbol Table symbol address a1 1000 0000 a2 1000 0004 a3 1000 0014 __start 0040 0000 loop 0040 0008 Memory map of data section address contents 1000 0000 0000 0003 1000 0004 0000 0010 1000 0008 0000 0010 1000 000c 0000 0010 1000 0010 0000 0010 1000 0014 0000 0005 Translate pseudo-instructions la $6, a2 loop: lw $7, 4($6) mul $9, $10, $7 b loop li $v0, 10 syscall lui $6, $6, 0x1000 ori $6, $6, 0x0004 lw $7, 4($6) mult $10, $7 mflo $9 b loop ori $v0, $0, 10 syscall Translate to machine code address lui $6, 0x1000 00400000 ori $6, 0x0004 00400004 lw $7, 4($6) 00400008 mult $10, $7 0040000c mflo $9 00400010 b loop 00400014 ori $v0, $0, 10 00400018 syscall 0040001c contents 3c06 1000 34c6 0004 8cc7 0004 012a 0018 0000 4812 1000 xxxx 3402 000a 0000 000c (lui) (ori) (lw) (mult) (mflo) (beq) (ori) (syscall) Resolve relative references address lui $6, 0x1000 00400000 ori $6, 0x0004 00400004 lw $7, 4($6) 00400008 mult $10, $7 0040000c mflo $9 00400010 b loop 00400014 ori $v0, $0, 10 00400018 syscall 0040001c contents 3c06 1000 34c6 0004 8cc7 0004 012a 0018 0000 4812 1000 fffd (-3) 3402 000a 0000 000c [0x400008 - (0x400014)]/4 = -12/4 = -3 = 0xfffd