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Instructor: John Barr

Machine-Level Programming II: Basics Comp 21000 : Introduction to Computer Organization & Systems Spring 2016. Instructor: John Barr * Modified slides from the book “Computer Systems: a Programmer’s Perspective”, Randy Bryant & David O’Hallaron , 2015. Machine Programming I: Basics.

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Instructor: John Barr

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  1. Machine-Level Programming II: BasicsComp 21000: Introduction to Computer Organization & Systems Spring 2016 Instructor: John Barr * Modified slides from the book “Computer Systems: a Programmer’s Perspective”, Randy Bryant & David O’Hallaron, 2015

  2. Machine Programming I: Basics • History of Intel processors and architectures • C, assembly, machine code • Assembly Basics: Registers, operands, move • Intro to x86-64

  3. Special memory areas: stack and heap Kernel virtual memory • Stack • Space in Memory allocated to each running program (called a process) • Used to store “temporary” variable values • Used to store variables for functions • Heap • Space in Memory allocated to each running program (process) • Used to store dynamically allocated variables User stack (created at runtime) Memory mapped region for shared libraries Run-time heap (created at runtime by malloc) Read/write segment (.data, .bss) Read-only segment (.init, .text, .rodata) Unused

  4. Example of Simple Addressing Modes void swap (long *xp, long *yp) { long t0 = *xp; long t1 = *yp; *xp = t1; *yp = t0; } swap: movq (%rdi), %rax movq (%rsi), %rdx movq %rdx, (%rdi) movq %rax, (%rsi) ret Note: if you look at the assembly code, it’s much more complicated; we’re ignoring some code to simplify.

  5. Understanding Swap() Memory Registers void swap (long *xp, long *yp) { long t0 = *xp; long t1 = *yp; *xp = t1; *yp = t0; } %rdi %rsi %rax %rdx Register Value %rdixp %rsiyp %rax t0 %rdx t1 swap: movq (%rdi), %rax # t0 = *xp movq (%rsi), %rdx # t1 = *yp movq %rdx, (%rdi) # *xp = t1 movq %rax, (%rsi) # *yp = t0 ret

  6. Understanding Swap() Memory Registers Address 0x120 123 %rdi 0x120 0x118 %rsi 0x100 0x110 %rax 0x108 %rdx 456 0x100 swap: movq (%rdi), %rax # t0 = *xp movq (%rsi), %rdx # t1 = *yp movq %rdx, (%rdi) # *xp = t1 movq %rax, (%rsi) # *yp = t0 ret

  7. Understanding Swap() Memory Registers Address 0x120 123 %rdi 0x120 0x118 %rsi 0x100 0x110 %rax 123 0x108 %rdx 456 0x100 swap: movq (%rdi), %rax # t0 = *xp movq (%rsi), %rdx # t1 = *yp movq %rdx, (%rdi) # *xp = t1 movq %rax, (%rsi) # *yp = t0 ret

  8. Understanding Swap() Memory Registers Address 0x120 123 %rdi 0x120 0x118 %rsi 0x100 0x110 %rax 123 0x108 %rdx 456 456 0x100 swap: movq (%rdi), %rax # t0 = *xp movq (%rsi), %rdx # t1 = *yp movq %rdx, (%rdi) # *xp = t1 movq %rax, (%rsi) # *yp = t0 ret

  9. Understanding Swap() Memory Registers Address 0x120 456 %rdi 0x120 0x118 %rsi 0x100 0x110 %rax 123 0x108 %rdx 456 456 0x100 swap: movq (%rdi), %rax # t0 = *xp movq (%rsi), %rdx # t1 = *yp movq %rdx, (%rdi) # *xp = t1 movq %rax, (%rsi) # *yp = t0 ret

  10. Understanding Swap() Memory Registers Address 0x120 456 %rdi 0x120 0x118 %rsi 0x100 0x110 %rax 123 0x108 %rdx 456 123 0x100 swap: movq (%rdi), %rax # t0 = *xp movq (%rsi), %rdx # t1 = *yp movq %rdx, (%rdi) # *xp = t1 movq %rax, (%rsi) # *yp = t0 ret

  11. Complete Memory Addressing Modes • Most General Form D(Rb,Ri,S) Mem[Reg[Rb]+S*Reg[Ri]+ D] • D: Constant “displacement” 1, 2, or 4 bytes • Rb: Base register: Any of 16 integer registers • Ri: Index register: Any, except for %rsp • Unlikely you’d use %rbp, either • S: Scale: 1, 2, 4, or 8 (why these numbers?) • Special Cases (Rb,Ri) Mem[Reg[Rb]+Reg[Ri]] D(Rb,Ri) Mem[Reg[Rb]+Reg[Ri]+D] (Rb,Ri,S) Mem[Reg[Rb]+S*Reg[Ri]] D(Rb,Ri,S) Mem[Reg[Rb]+S*Reg[Ri]+D]

  12. Address Computation Examples %rdx 0xf000 %rcx 0x100

  13. Address Computation Examples %edx 0xf000 %ecx 0x100 0xf000+ 0x8 0xf008 0xf000 + 0x100 0xf100 0xf000 + 4*0x100 0xf400 2*0xf000 + 0x80 0x1e080

  14. Address Computation Instruction lea = load effective address • leaqSrc,Dest • Src is address mode expression • Set Dest to address denoted by expression • Format • Looks like a memory access • Does not actually access memory • Rather, calculates the memory address, then stores in a register • Uses • Computing addresses without a memory reference • E.g., translation of p = &x[i]; • Computing arithmetic expressions of the form x + k*y • k = 1, 2, 4, or 8.

  15. Example Converted to ASM by compiler: long m12(long x) { return x*12; } leaq (%rdi,%rdi,2), %rax ;t <- x+x*2 salq $2, %rax ;return t<<2 ; or t * 4

  16. Address Computation Instruction Assume %rax holds value x and %rcx holds value y * Note the leading comma in the next to last entry

  17. Address Computation Instruction • %rdx x + 6 • %rdx x + y • %rdxx + (y * 4) • %rdxx + (x * 8) + 7 = (x * 9) + 7 • %rdx (x * 4) + 10 • %rdxx + (y * 2) + 9 Assume %rax holds value x and %rcx holds value y * Note the leading comma in the next to last entry

  18. Machine Programming II: Summary • History of Intel processors and architectures • Evolutionary design leads to many quirks and artifacts • C, assembly, machine code • New forms of visible state: program counter, registers,… • Compiler must transform statements, expressions, procedures into low-level instruction sequences • Assembly Basics: Registers, operands, move • The x86-64 move instructions cover wide range of data movement forms • Intro to x86-64 • A major departure from the style of code seen in IA32

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