Lesson 17 · 16-bit increment - Build a Game Boy Emulator

16-bit increment

Today you implement 16-bit INC/DEC - stepping a whole register pair up or down which, unlike their 8-bit cousins, touch no flags at all. It rounds out the arithmetic core and gives you the pointer-stepping the memory chapter will lean on.

The goal

Implement INC BC and DEC BC, which step a register pair up or down without touching any flags.

Start here - the target

TO DO

Scenario: Incrementing and decrementing a pair, with no flags touched

GivenBC is 0x00FF and all flags are clear

WhenINC BC (opcode 0x03) runs

ThenBC is 0x0100

Andevery flag is still clear - 16-bit INC affects no flags

Andfrom BC = 0x0100, DEC BC (opcode 0x0B) gives BC = 0x00FF with every flag still clear

Andit wraps in 16 bits - from BC = 0xFFFF, INC BC gives BC = 0x0000; from BC = 0x0000, DEC BC gives BC = 0xFFFF (flags still clear)

Background

INC BC (opcode 0x03) adds one to a whole register pair, carrying naturally from the low byte into the high byte - 0x00FF becomes 0x0100. Unlike its 8-bit cousin from lesson 15, the 16-bit INC/DEC affect no flags whatsoever, because the pair is usually a pointer being walked and the program does not want its status bits disturbed for that.

With this you have a complete arithmetic core: 8- and 16-bit add and subtract, carries, logic, compare, and counting. If you want a victory lap, hand-assemble a short straight-line program - a few adds and increments - and check the final registers against what you worked out on paper; that’s an optional stretch, not required today. Real loops need the conditional jumps of the next chapter, but the math underneath them is done.

Make it work

case 0x03: // INC BC - no flags touched at all
    c.SetBC(c.BC() + 1)
    return 8
case 0x0B: // DEC BC - likewise leaves every flag alone
    c.SetBC(c.BC() - 1)
    return 8