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Re: Typo in ProDOS refTechMan

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Groovy hepcat Michael J. Mahon was jivin' in comp.sys.apple2.programmer
on Fri, 1 Sep 2023 04:54 am. It's a cool scene! Dig it.

> Peter 'Shaggy' Haywood <phaywood@alphalink.com.au> wrote:
>> Groovy hepcat Michael J. Mahon was jivin' in
>> comp.sys.apple2.programmer on Mon, 28 Aug 2023 04:45 pm. It's a cool
>> scene! Dig it.
>> 
>>> Jeff Blakeney <CUTjeffrey_blakeney@yahoo.ca> wrote:
>>> 
>>>> In other words, the SYSCALL is the address of the JSR but it isn't
>>>> the address that gets pushed onto the stack when the JSR is
>>>> executed.
>>> 
>>> Well, the details are: JSR pushes the address of the JSR opcode plus
>>> *two* on the stack, and the RTS pops the address and adds *one* to
>>> get the return PC value.
>> 
>>   Not quite. What happens is that the first byte of the instruction
>>   (the
>> op code) is read and, depending on the particular code, either 0, 1
>> or 2 operand bytes are read in. In the case of a JSR, 3 bytes are
>> read in total, the op code and a 2 byte address operand. The program
>> counter is updated as each byte is read in.
>>   Next, the operation is carried out. For JSR that means the current
>> value of the PC, which now contains the address of the next
>> instruction (or whatever occupies the memory following the JSR,
>> including operand) is pushed on the stack before execution branches
>> to the new location.
>>   An RTS instruction just pulls the address off the stack and bungs
>>   it
>> into the PC so that execution can take up from there.
>> 
>>> This doesn’t change the need to add 3 to the stacked value to skip 3
>>> bytes of in-line data, but it does mean that the stacked address
>>> points at the high byte of the JSR, not the first byte of the
>>> in-lined data.
>> 
>>   No, it is the address of the next instruction. You're right about
>>   not
>> changing the need to add 3 to the address. It's just that the address
>> is that of the following op code..., which before 3 is added, in this
>> case, isn't an op code but data.
>>   
> 
> Before posting, you should have RTFM. ;-)
> 
> That is NOT how JSR/RTS works.
> 
> You can “pretend” that it works like that as long as you never use (or
> create) the return data that is pushed on the stack.

  After doing some reading, I see that you are quite correct. I bow to
superior knowledge and would like to say, in the words of Maxwell
Smart, "Sorry about that, Chief!"

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Typo in ProDOS refTechMan Colin Leroy-Mira <colin@colino.net> - 2023-08-22 22:19 +0200
  Re: Typo in ProDOS refTechMan Jerry Penner <jerry+a2@jpen.ca> - 2023-08-22 22:49 -0600
    Re: Typo in ProDOS refTechMan Colin Leroy-Mira <colin@colino.net> - 2023-08-23 08:23 +0200
      Re: Typo in ProDOS refTechMan Oliver Schmidt <ol.sc@web.de> - 2023-08-23 07:38 +0000
        Re: Typo in ProDOS refTechMan Colin Leroy-Mira <colin@colino.net> - 2023-08-23 15:22 +0200
          Re: Typo in ProDOS refTechMan Jerry Penner <jerry+a2@jpen.ca> - 2023-08-24 00:06 -0600
            Re: Typo in ProDOS refTechMan Jeff Blakeney <CUTjeffrey_blakeney@yahoo.ca> - 2023-08-24 08:10 -0400
              Re: Typo in ProDOS refTechMan Michael J. Mahon <mjmahon@aol.com> - 2023-08-28 06:45 +0000
                Re: Typo in ProDOS refTechMan Jeff Blakeney <CUTjeffrey_blakeney@yahoo.ca> - 2023-08-28 08:28 -0400
                Re: Typo in ProDOS refTechMan Michael J. Mahon <mjmahon@aol.com> - 2023-08-31 18:54 +0000
                Re: Typo in ProDOS refTechMan Peter 'Shaggy' Haywood <phaywood@alphalink.com.au> - 2023-08-31 13:30 +1000
                Re: Typo in ProDOS refTechMan Michael J. Mahon <mjmahon@aol.com> - 2023-08-31 18:54 +0000
                Re: Typo in ProDOS refTechMan Antoine Vignau <ntn.vignau@gmail.com> - 2023-09-07 12:41 -0700
                Re: Typo in ProDOS refTechMan Peter 'Shaggy' Haywood <phaywood@alphalink.com.au> - 2023-09-05 12:14 +1000
  Re: Typo in ProDOS refTechMan qkumba <peter.ferrie@gmail.com> - 2023-08-28 10:23 -0700
    Re: Typo in ProDOS refTechMan Antoine Vignau <ntn.vignau@gmail.com> - 2023-08-28 12:29 -0700

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