I'm working on a Gameboy Emulator, and I've reached a point in the ROM where I get opcode 0xD1 (pop DE off stack) but the stack is empty (no values have been pushed onto it). All unknown opcodes return an error, and all other instructions seem to be working fine.
Is it an error in my programming, the ROM, or is this just a quick way for the program to set DE to 0x0000?
Even if no value has been PUSHed to the stack, POP will retrieve the value stored at the address in SP to the specified register pair, and SP will be incremented by 2.
In your example, if SP has been initialized to, say wD000, and that the WRAM is initialized to 0 beforehand, POP DE would effectively load 0 to DE, and increment the Stack Pointer by 2.
21 00 C0 ld hl,C000 ;Start of WRAM
01 FF 1F ld bc,1FFF ;Length of WRAM
AF xor a ;a = 0
22 ldi (hl),a ;Blanks WRAM
0B dec bc
78 ld a,b
B1 or c
20 F9 jr nz,0158 ;Loops until WRAM is cleared
21 00 D0 ld hl,D000
F9 ld sp,hl ;SP = 0xD000
D1 pop de ;de = 0x0000, SP = 0xD002
Also, please note that the CALL instruction pushes the return address to the stack, and decrements SP by 2. In the same way, RET retrieves the address from the stack, and increases SP by 2.