go-delve/delve · warning · ErrUnknownRegister

unknown register

Error message

unknown register

What it means

ErrUnknownRegister is returned when a register is requested by name that the architecture's register set does not define. GetReg/Get (and DWARF register name resolution via getAsmRegister) look up names in the arch-specific register table and return this sentinel when the name is not present.

Source

Thrown at pkg/proc/registers.go:50

	Name string
	Reg  *op.DwarfRegister
}

// AppendUint64Register will create a new Register struct with the name and value
// specified and append it to the `regs` slice.
func AppendUint64Register(regs []Register, name string, value uint64) []Register {
	return append(regs, Register{name, op.DwarfRegisterFromUint64(value)})
}

// AppendBytesRegister will create a new Register struct with the name and value
// specified and append it to the `regs` slice.
func AppendBytesRegister(regs []Register, name string, value []byte) []Register {
	return append(regs, Register{name, op.DwarfRegisterFromBytes(value)})
}

// ErrUnknownRegister is returned when the value of an unknown
// register is requested.
var ErrUnknownRegister = errors.New("unknown register")

type flagRegisterDescr []flagDescr
type flagDescr struct {
	name string
	mask uint64
}

var mxcsrDescription flagRegisterDescr = []flagDescr{
	{"FZ", 1 << 15},
	{"RZ/RN", 1<<14 | 1<<13},
	{"PM", 1 << 12},
	{"UM", 1 << 11},
	{"OM", 1 << 10},
	{"ZM", 1 << 9},
	{"DM", 1 << 8},
	{"IM", 1 << 7},
	{"DAZ", 1 << 6},
	{"PE", 1 << 5},

View on GitHub (pinned to a23773e6c3)

Solutions

1. Use a register name valid for the target architecture (list them via the appropriate regs/API for the arch)
2. Fix the register name spelling and case in your script/command
3. Check runtime.GOARCH of the debugged binary before naming registers

Example fix

// before
val, err := regs.Get("eax") // on arm64
// after
val, err := regs.Get("x0") // arch-appropriate register name

// check arch before naming registers
var reg string
switch runtime.GOARCH {
case "arm64":
    reg = "x0"
case "amd64":
    reg = "rax"
case "386":
    reg = "eax"
}

val, err := regs.Get(name)
if errors.Is(err, proc.ErrUnknownRegister) {
    return fmt.Errorf("register %q not valid for %s", name, runtime.GOARCH)
}

When it happens

Trigger: Evaluating an expression that references a register name not valid for the target architecture (e.g. 'xmm0' on arm64, 'r15' on 386); RPC calls to GetReg with misspelled names; DWARF CFA/rules referencing registers the backend does not model.

Common situations: Scripts or IDE plugins hardcoding x86 register names while debugging arm64; cross-arch debugging sessions; using DWARF files that reference pseudo-registers the backend lacks.

Related errors

• read out of bounds
• could not read %d bytes from register %d (size: %d), also er
• could not read %d bytes from register %d (size: %d)
• bad address size
• ErrStackUnderflow

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/871ff8c84f12d5d9. Report an issue: GitHub.