Assembly substraction

Question

Hello i started to learning basic 6502 assembly. Unfortunately i encountered first problem. I put code here and my opinion's what's doing what.

> LDA #$5            <-- we put value $5 in register A(accumulator) STA
> STA$01            <--  we store vaule of A in memory $01 LDA #$8          
> LDA #$8           <--  we load value $8 to A  SBC $01         
> SBC $01          <---  accumulator a number stored in $01;

Now, when i am using this site: http://skilldrick.github.io/easy6502/#registers

After going through all steps my accumulator is not $3. It is $2. It seems that i dont understand sth. Could u point where i do a mistake? I would be grateful for help.

Answer 1

The C in SBC stands for Carry. If the Carry flag is clear, SBC subtracts an extra 1.

Always do SEC before a lone SBC, or before the first in a series of SBC's implementing a multi-byte subtraction, the same way you do CLC before an ADC or series of ADC's.