This paper http://www.ncsu.edu/wcae/WCAE1/hanson.pdf contains a much more detailed block diagram for the 6502 than is present in any of the data sheets (see last page). You can see the same image here http://www.pagetable.com/?p=39 as well - but a bit quicker.

You can see from the diagram how the ALU uses a inverters on the second input to ones-complement before addition to generate a subtraction, and has a completely seperate module for decimal adjustment.

I can feel some VHDL coming on.

**Edit Added alternative link.

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

Cool; thanks! If anyone's looking for the huge schematic of the die, please see this thread.

Greg

I knew about the inverters just by the fact that you have to SEC before SBC.

Upon accessing the link, however, I get permission denied message.

I just tested the link and its working.

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

That's very strange; must have been having some webserver problems, as I can now download it. Thanks.

Hanson's block diagram is now found on pagetable's article about illegal opcodes.

(this topic is the top hit for 6502 block diagram)

Cheers
Ed