Lesson 05 · The flags register - Build a Game Boy Emulator

The flags register

Today you turn register F into the CPU's status flags, four independent bits that record whether a result was zero, negative, or overflowed. Every arithmetic instruction from here on reports its outcome through these same four flags.

The goal

Treat the top four bits of register F as the Z, N, H, and C flags, each settable and clearable independently.

Start here - the target

TO DO

Scenario: Setting and clearing the zero flag

Givenregister F is 0x00

Whenthe Z flag is set true

ThenF is 0x80

Andsetting the C flag true as well makes F 0x90, while the low nibble stays 0

Background

The F register is not a general-purpose byte - it is the CPU’s status flags. Only its top four bits mean anything: bit 7 is Z (zero), bit 6 is N (subtract), bit 5 is H (half-carry), and bit 4 is C (carry). The bottom four bits are always zero and cannot be set.

These four flags are how instructions record what happened - a result was zero, an addition overflowed - and how later instructions decide whether to branch. Model them as a mask over F now, keeping the low nibble forced to zero, and every arithmetic lesson that follows just flips the right bit.

Make it work

// bits 3..0 always read 0
const (
    FlagZ = 0x80 // bit 7 - zero
    FlagN = 0x40 // bit 6 - subtract
    FlagH = 0x20 // bit 5 - half-carry
    FlagC = 0x10 // bit 4 - carry
)
func (r *Registers) SetFlag(mask uint8, on bool) {
    if on { r.F |= mask } else { r.F &^= mask }
    r.F &= 0xF0 // keep the low nibble zero
}