// main.go
package main

import (
    "fmt"
)

func readmsr(register int64) int64

func main() {
    fmt.Println(readmsr(0xC0000080))
}

// intrinsics_amd64.s
// readmsr(register int64) int64
TEXT ·readmsr(SB), $0-24
    RDMSR register+0(FP), AX
    MOVQ AX, ret+16(FP)
    RET

go build -v .
_/gopath/go/workspace/temp
# _/gopath/go/workspace/temp
asm: invalid instruction: 00000 (/gopath/go/workspace/temp/instrinsics_amd64.s:3)   RDMSR   register+8(FP), AX
asm: assembly failed