Emulating a CPU in C++ #26 (6502) - Subtract With Carry

In this video i get SBC "Subtract With Carry" instruction working in the 6502 emulator. A lot of the content here is based on having a working ADC "Add With Carry", so if you haven't seen the ADC videos or don't know how ADC works then you should watch that first.
Links:
Source for this project: github.com/davepoo/6502Emulator
Google Test: github.com/google/googletest
6502 Information: www.obelisk.me.uk/6502/
Timestamps
0:00 SBC definition
2:00 Whiteboard theory
11:10 test 0 + 0 = 0
16:16 Implementing SBC Absolute
20:18 0 - 1= -1 test
24:36 0 - 1 - C = -2 test
27:47 -ve signed overflow test
29:52 +ve signed overflow test
32:50 0 - 0 - C = -1 test
34:20 Subtract 2 +ve numbers test
36:00 Tidy up and closing comments

[@twobob]

Solid. Agreed, hardware must be like that

[@CZghost]

A - M - (1 - c) This is actually very poorly written, but it is explained quite correctly. It should be written like this: A + (~M) + (~C), where "~" means logical negation of the register and the flag (it could also be written as !C) Then your code is incorrect, because in real hardware, the carry flag isn't taken as is, it is actually negated, as well as the input to the ALU (negation is controlled by a microinstruction that tells the CPU to flip the bits - the actual control logic of the CPU that decodes the opcodes). So in reality you should actually copy the code over and change the addition of the operand and the carry into addition of their negation (so Operand becomes ~Operand, bitwise NOT so all of the bits are flipped, and Flags.C becomes !Flags.C, because it is only one bit you can use boolean NOT). Or interpret it into the existing ADC function so you actually set an operation to it and it decides based on the operation (and possibly rename the function). That would have much closer function to the real hardware. This mistake might actually lead to bugs and errors in otherwise fully working program code.

[@thomasfriedrich2001]

operator ~ is the bitwise complement, not the negative / two's complement.

[@DavePoo]

Yeah, i think i called it "bitwise negation"

[@CostasB-jv1ko]

So we make the subtraction using the addition "circuit" based on the formula (Acc + Memory(1's complement) + Carry) ? Super confusing ..... Let's say Acc holds 0x01 and i want to subtract 0x01 from it, the Carry flag should be set to TRUE to get the result of 0x00. LDA #$0x01 SEC SBC #$0x01;Now Acc is cleared Am i missing something?

[@DavePoo]

The result will be zero but the carry flag will be TRUE after the operation.

[@TheJamieRamone]

A - M - (1 - c) = A - M - 1 + c