src/drivers/cpu_asm.s

/*
 * cpuid -- nsc has no inline asm (same gap as every other "needs a
 * real instruction" driver here, see io.s's own note), so this is the
 * one hand-written wrapper cpu.nsc calls through.
 *
 * cpuid(leaf: %edi, out: %rsi): sets eax=leaf, ecx=0 (no caller here
 * ever needs a real subleaf), executes cpuid, and writes eax/ebx/ecx/
 * edx as four zero-extended 8-byte words at out[0]/out[8]/out[16]/
 * out[24] -- the same "every value crossing the asm/nsc boundary is a
 * full 8-byte word" convention this whole kernel uses (nsc's `*p` is
 * always a full 8-byte load/store, never 32-bit-granular). rbx is
 * callee-saved per sysv and cpuid clobbers it, so it's the only
 * register that needs saving here -- rsi (out) is never touched by
 * cpuid itself.
 */
.section .text
 
.global cpuid
.type cpuid, @function
cpuid:
	push %rbx
	movl %edi, %eax
	xorl %ecx, %ecx
	cpuid
	movq %rax, 0(%rsi)
	movq %rbx, 8(%rsi)
	movq %rcx, 16(%rsi)
	movq %rdx, 24(%rsi)
	pop %rbx
	ret