Question

On Gameboy Z80, exactly how does the LD HL,(SP+e) operation affect H and C flags? (Half-carry + carry)

Reference: http://www.devrs.com/gb/files/opcodes.html

Answer 1

I realise this is an old question but I had a similar problem a while ago and would just like to add my solution as there is absolutely no documentation or open-source emulator that does it correctly to my knowledge. Took me some actual debugging on a real gameboy to find the solution.

For both 16bit SP + s8 (signed immediate) operations:

the carry flag is set if there’s an overflow from the 7th to 8th bit.

the half carry flag is set if there’s an overflow from the 3rd into the 4th bit.

I found it easier to do the behaviour separately for both a positive and negative signed immediate (Lua):

local D8 = self:Read(self.PC+1)
local S8 = ((D8&127)-(D8&128))
local SP = self.SP + S8 

if S8 >= 0 then
    self.Cf = ( (self.SP & 0xFF) + ( S8 ) ) > 0xFF
    self.Hf = ( (self.SP & 0xF) + ( S8 & 0xF ) ) > 0xF
else
    self.Cf = (SP & 0xFF) <= (self.SP & 0xFF)
    self.Hf = (SP & 0xF) <= (self.SP & 0xF)
end