Add notes from 2023-08-07

2023-08-07 10:37:59 +01:00 · 2023-08-07 10:37:59 +01:00 · de69180456
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+# Dev notes — 2023-08-07
+
+## Carry vs overflow
+
+[Understanding the difference between overflow and carry flags](https://stackoverflow.com/questions/69124873/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 <mark>carry is unsigned</mark>  
+and <mark>overflow is signed</mark>
+
+(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
+
+- <mark>TODO:</mark> 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?