2023-08-29 - Add info on how overflow flag should behave

2023-08-29--dev-notes.md

@@ -1,5 +1,65 @@
 # Dev notes — 2023-08-29
 
+## Overflow flag
+
+This looks like a better (simpler to understand) resource: http://c-jump.com/CIS77/CPU/Overflow/lecture.html#O01_0090_signed_overflow
+
+> When two signed 2's complement numbers are added, overflow is detected if:
+> 1. both operands are positive and the sum is negative, or
+> 2. both operands are negative and the sum is positive.
+
+> Notice that overflow occurs only when  
+> `CARRYin ≠ CARRYout`  
+> or simply  
+> `V = Cin XOR Cout`  
+> where `V` is the overflow signal.
+
+And http://teaching.idallen.com/dat2343/11w/notes/040_overflow.txt:
+
+> The rules for turning on the overflow flag in binary/integer math are two:
+>
+> 1. If the sum of two numbers with the sign bits off yields a result number
+   with the sign bit on, the "overflow" flag is turned on.
+>
+>    0100 + 0100 = 1000 (overflow flag is turned on)
+>
+> 2. If the sum of two numbers with the sign bits on yields a result number
+>    with the sign bit off, the "overflow" flag is turned on.
+> 
+>    1000 + 1000 = 0000 (overflow flag is turned on)
+> 
+> Otherwise, the overflow flag is turned off.
+>  * 0100 + 0001 = 0101 (overflow flag is turned off)
+>  * 0110 + 1001 = 1111 (overflow flag is turned off)
+>  * 1000 + 0001 = 1001 (overflow flag is turned off)
+>  * 1100 + 1100 = 1000 (overflow flag is turned off)
+> 
+> Note that you only need to look at the sign bits (leftmost) of the three
+> numbers to decide if the overflow flag is turned on or off.
+> 
+> If you are doing two's complement (signed) arithmetic, overflow flag on
+> means the answer is wrong - you added two positive numbers and got a
+> negative, or you added two negative numbers and got a positive.
+> 
+> If you are doing unsigned arithmetic, the overflow flag means nothing
+> and should be ignored.
+
+...
+
+> Overflow in two's complement may occur, not when a bit is carried out
+of the left column, but when one is carried into it and no matching
+carry out occurs. That is, overflow happens when there is a carry into
+the sign bit but no carry out of the sign bit.
+> 
+> The OVERFLOW flag is the XOR of the carry coming into the sign bit (if
+any) with the carry going out of the sign bit (if any).  Overflow happens
+if the carry in does not equal the carry out.
+
+And https://stackoverflow.com/a/8966863:
+
+> The signed overflow flag value, however, must be the same for both A-B and A+(-B) because it depends on whether or not the result has the correct sign bit and in both cases the sign bit will be the same.
+
+
 ## Short-term goals:
 
 - [ ] **Update main TODO list**
@@ -20,7 +80,8 @@
 
 ***
 
-- [ ] **fix overflow flag**
+- [x] **fix overflow flag**
+- [ ] **add 'dry run' mode to assembler - print debug info but don't write to file**
 - [ ] implement 'jmp ($FF)' on startup
   - [ ] implement ROM