Assembly Language

  • Low level
  • Mnemonics, Label 用以表示CPU指令
  • 組譯(assemble)成CPU指令

Instruction set design

  • Register
  • Data Bus / Address Bus
  • Operation / Operand counts
  • Literal value
  • Flow control
  • CISC / RISC

Addressing mode

  • Direct
    • ldr r0, MEM
  • Immediate
    • mov r0, #1
  • Register direct
    • mov r0, r1
  • Register indirect
    • ldr r0, [r1, #4] (pre-indexed)
    • ldr r0, [r1, #4]! (auto-indexed)
    • ldr r0, [r1], #4 (post-index, auto post-increment)
    • ldr r0, [r1, r2 LSL #2] (with scaling)
  • PC relative
    • ldr r0, [PC, #offset]

System software

  • Assembler
    • Directive 以".“作為開頭
    • Pseudo instruction : Alias
    • 轉換mnemonics, label以及literal
  • Relocation
    • 相對位址

Building of GNU

  • Build給amd64, target=arm

Segments

  • 參見 ELF cheatsheet
  • .text : code, r-x
  • .data : data, rw- / r-- (rodata/rdata)
  • .bss : uninitialized data, rw-
  • NX bit

ARM architecture

  • 採用RISC

  • Endianness (不重要)

  • Register

    • 通用 : r0 ~ r10 (NR只能放在 r7)
    • FP : Frame Pointer(x86 RBP), r11
    • IP : Intra Pointer, r12
    • SP : Stack Poiter, r13
    • LR : Link Register, r14
    • PC : r15
    • CPSR/APSR : Flags, 只有做比較/S做結尾的指令會更新flag

  • Conditional Execution

    • CPU不會知道signed/unsigned/negative, 一切都依賴設定好的Flags
    • B / BL(link) / BX(exchange) / BLX
    • EQ : ==
    • NE : !=
    • GT/GE : > / >=
    • LT/LE : < / <=
    • HS/LS : Higher / Lower (or same)
    • HI/LO : Higher / Lower
    • MI : +
    • PL : -
    • VS/VC : overflow / no overflow

  • Addressing mode

    • i++ / ++i的區別
    • Pre-Index : 不會更新
      • ldr r0, [r1, #1]
    • Auto-Index : 先更新, 接著做指令
      • ldr r0, [r1, #1]!
    • Post-Index : 做完才更新 (這裡的r0會取[r1])
      • ldr r0, [r1], #1

  • Load / Store

    • 參見 context switch
    • STM/LDM 指令
      • IA : 遞增
      • DB : 遞減
      • stm r0, {r1, r2} : [r0+4]=r1, [r0]=r2
      • ltm r0, {r1, r2} : r1=[r0], r2=[r0+4]
    • push / pop