The use of = sign in ARM assembly

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Load constants

All AArch64 instructions are 32-bits long…

ARM instruction 只有 32 bits 長，需要借助 pseudo instruction: adr, ldr 的輔助，例如載入常數

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ldr rd, =constant # rd=32-bit constant
adr rd, label     # rd=32-bit relative address

ldr 會依據情況轉換成 mov, mvn, ldr [pc, #offset] 等指令，= 用意是從 literal pool 找出該常數，若沒有就放入 literal pool。

  • Source code

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    ldr     r0, =0xff
    ldr     r1, =0xffffffff
    ldr     r2, =0xf00000ff

  • Disassembly

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    mov     r0, #0xff
    mvn     r1, #0x00
    ldr     r2, [pc, #offset]

adr 在 entry.S 用意是將 sys_call_table 地址載入到 stbl，再來藉由 ldr x16, [stbl, scno, lsl #3] 載入 syscall 地址。

ADR accepts a PC-relative expression, that is, a label with an optional offset where the address of the label is relative to the PC.

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sc_nr	.req	x25					// number of system calls
scno	.req	x26					// syscall number
stbl	.req	x27					// syscall table pointer

el0_svc:
	adr	stbl, sys_call_table			// load syscall table pointer
	uxtw	scno, w8				        // syscall number in w8
	mov	sc_nr, #__NR_syscalls
	bl	enable_irq
	cmp     scno, sc_nr           // check upper syscall limit
	b.hs	ni_sys

	ldr	x16, [stbl, scno, lsl #3]	// address in the syscall table
	blr	x16					              // call sys_* routine
	b	ret_from_syscall
ni_sys:
	handle_invalid_entry 0, SYSCALL_ERROR
ret_from_syscall:

Reference

Addressing modes

The address for a load or store is formed from two parts: a value from a base register, and an offset.

The offset takes one of three formats:

  • Immediate
  • Register
  • Scaled register

The offset and base register can be used in three different ways to form the memory address.

  • Offset
  • Pre-indexed
  • Post-indexed

直接看上課簡報認識指令