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Dev notes — 2023-08-07

Carry vs overflow

Understanding the difference between overflow and carry flags

Carry indicates the result isn't mathematically correct when interpreted as unsigned, overflow indicates the result isn't mathematically correct when interpreted as signed.

1111 + 0001 = 0000 should set carry (15 + 1 = 0 is false) and clear overflow (-1 + 1 = 0 is true).

0111 + 0010 = 1001 should clear carry (7 + 2 = 9 is true) and set overflow (7 + 2 = -7 is false).

1001 + 1001 = 0010 should set both (9 + 9 = 2 and -7 + -7 = 2 are both false).

so carry is unsigned
and overflow is signed

(which is what i've got, good)

add more flags + change flag ops

flags register

Replace the current 'Carry Flag' with a Flags Register.

Here's a sketch for the bit pattern:

hi bit

0
0
0
0

0 ? negative
0 ? zero
0 overflow
0 carry

lo bit

cf. 6502:

NV-BDIZC
7: Negative, 6: Overflow, 5: none, 4: Break, 3: Decimal, 2: Interrupt disable, 1: Zero, 0: Carry

flag opcodes

replace CHP and CFC with FHP and FTG
FHP n: hop if flag n is set
- eg: FHP 0 = hop if carry flag set
- eg: FHP 1 = hop if overflow flag set
- to keep it simple, we're just giving each flag a number, not fussing with bitmasking or anything
FTG n: toggle flag n on/off
- eg: if Carry is on, FTG 0 turns it off
- eg: if Overflow is off, FTG 1 turns it on

FHP and FTG can be combined to create set flag and unset flag routines:

@set_carry:
  FHP 0
   FTG 0

If Carry is on when this is called, then FTG is skipped and Carry remains set. Otherwise, FTG sets carry.

; call with a return address stored at $01

@unset_carry:
  FHP 0      ; 1
   FHP 0     ; 2
  FTG 0      ; 3
  JMP ($01)  ; jump back to caller

If Carry is on when this is called, then the execution is: 1, 3, and Carry is turned off.

If Carry is off, then the execution is: 1, 2, (hop over 3; Carry is still off), jump back to caller.

Think about a subroutine stack?

Maybe?

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