Extracranial
ENKO

ARM Architecture

Registers

  • Register names can either be uppercase or lowercase, but no mixing.
  • 64 bits: x0, x1, ... , x30 (extended)
  • 32 bits: w0, w1, ... , w30 (word)
  • Immediate values have # prefix.
  • Brackets [] is the value in the memory.
  • If x1 holds 1024, the following instruction will load Memory[1024] to x0.
LDR x0, [x1]

Special Registers

  • Virtual Zero Registers: xzr, wzr: 0 when read, ignore writes.
  • Stack Pointer sp errors if not aligned to a multiple of 16 bytes.
  • Frame pointer fp is an alias of x29. Same as rbp (Base Frame Pointer)
  • Link Register lr is an alias of x30. Holds the return address used by ret.
    • Return addresses does not automatically get pushed to stack.

Arithmetics

  • Always operate between registers and immediate values.
    • Do not operate directly with memory.
  • Takes three operands. OP destination, A, B means destination = A OP B
  • Can take flexible operands
    • bitwise operations (add, orr, eor, mvn) and add, sub, and mov can do optional shift or rotation of the last element.
    • add x1, x2, x3, lsl 32 means x1 = x2 + (x3 << 32).
    • add w1, w2, w3, ror 8 means w1 = w2 + (w3 >>> 8).

Memory Load and Store

  • Unlike x86, operations do not need size suffixes. They are inferred.
  • Has Register Pair Load, Store mode.
    • ldr x0, x1, [x2]: x2 → x0, x2+8 → x1
  • mov only between registers.
  • By default, transfers are between 4 bytes w register or 8 bytes x registers.
  • To change in size, use a suffix for zero-extended mode or sign-extended mode.
    • 1 byte read: b (zero-extend), sb (sign-extend)
    • 2 bytes read: h (zero-extend), sh (sign-extend)
    • ldrb w0, [x1] read 1 byte at Memory[x1], zero-extend to 4 bytes, put to w0

Addressing Modes

NameExampleAddress UsedSide-Effect
Baseldr x1, [x2]Memory[x2]
Base + Offsetldr x1, [x2, 16]Memory[x2+16]
Pre-indexedldr x1, [x2, 16]!Memory[x2+16]x2 = x2 + 16, just like ++i in C
Post-indexedldr x1, [x2], 16Memory[x2]x2 = x2 + 16, just like i++ in C
Base + Registerldr x1, [x2, x3]Memory[x2 + x3]
Scaledldr x1, [x2, x3, lsl 2]Memory[x2 + (x3 << 2)]
Sign-Extendedldr x1, [x2, w3, sxtw]Memory[x2 + SignExtend(w3)]
Zero-Extendedldr x1, [x2, w3, uxtw]Memory[x2 + ZeroExtend(w3)]
Sign-Extended Scaledldr x1, [x2, w3, sxtw 2]Memory[x2 + (SignExtend(w3) << 2)]

Calling Conventions

  • Arguments. x0, ... , x7 then saved on the stack.
  • Return Value gets stored in x0.
  • Caller-Saved Registers. x0 to x18.
  • Callee-Saved Registers. x19 to x29.
  • Callee Saves Link Register (lr, alias of x30) if it invokes any procedures.
  • Branch with Link (bl) sets lr, alias of x30 to PC + 4.
  • Return ret by default jumps to Mem[x30] but can also use ret x? for any other registers.

References