It seems to me that if you don't increment PC at step 2, you can't fetch the high address byte from memory at step 6. JSR is 3 bytes, so three fetches from memory. I could read that code wrong anyway. On Wed, Sep 19, 2018 at 4:43 PM <Ruud@baltissen.org> wrote: > > Hallo allemaal, > > I'm building two TTL CPUs right now. For those who have no idea what I mean, have a look here: http://homebrewcpuring.org/ > Please have a look at: http://www.6502.org/users/dieter/m02/m02.htm and http://www.6502.org/users/dieter/m02/system.jpg > Notice that this computer only emulates the C64 more or less. > > My first TTL CPU has no Instruction Decoder which means every action that the CPU has to do has to be programmed. Advantage: you need less parts, disadvantage: you need (roughly) about ten times as much memory and it is twenty times as slow. > > The second one has an Instruction decoder (ID) and should be able to emulate the 6502, although not cycle exact. The ID is made from seven FlashRAMs and needs to be filled with data of course. For this I am writing a program that analyses every step of every instruction and then sets or resets the according bits of the involved FRAM. > Now at this moment I am dealing with JSR. Next is the text for JSR from 64doc.txt found on Bo Zimmers site: > > # address R/W description > --- ------- --- ------------------------------------------------- > 1 PC R fetch opcode, increment PC > 2 PC R fetch low address byte, increment PC > 3 $0100,S R internal operation (predecrement S?) > 4 $0100,S W push PCH on stack, decrement S > 5 $0100,S W push PCL on stack, decrement S > 6 PC R copy low address byte to PCL, fetch high address > byte to PCH > > What I read is: at step 1 and 2 the Program Counter is increased and thus points to the next instruction at step 4. So this is the address that is pushed to the stack at step 4 and 5. > But IMHO the PC is only incremented once before pushed to the stack. When RTS is executed, the stored address is read, written into the PC and increased according this document. Which prooves the incremental at step 2 is wrong. > > Your comment, please. > > > -- > > Kind regards / Met vriendelijke groet, Ruud Baltissen > www.Baltissen.org > > > > > >Received on 2018-09-19 17:02:02
Archive generated by hypermail 2.2.0.
