Typo in ProDOS refTechMan

[Colin Leroy-Mira]

Hi everyone,

I think there's a typo in the ProDOS Technical Reference Manual at
http://www.easy68k.com/paulrsm/6502/PDOS8TRM.HTM#4.2

In fact, "Upon completion of the call, the MLI returns to the address
of the JSR plus 6" - not 3.

Unless I'm mistaken.

HTH,
--
Colin
https://www.colino.net/

[Jerry Penner]

Colin Leroy-Mira <co...@colino.net> writes:

> Hi everyone,
>
> I think there's a typo in the ProDOS Technical Reference Manual at
> http://www.easy68k.com/paulrsm/6502/PDOS8TRM.HTM#4.2
>
> In fact, "Upon completion of the call, the MLI returns to the address
> of the JSR plus 6" - not 3.
>
> Unless I'm mistaken.
>
> HTH,

I think you're right. What ProDOS does is add 3 bytes to the return
address to pass over the command-number and command-list-address, but
the effect is that execution continues 6 bytes after the start of the
JSR instruction. I have read that page many times and never noticed
that, probably because I understood what was going on with the
return-address.

Nice catch.

--
--
Jerry jerry+a2 at jpen.ca

[Colin Leroy-Mira]

Hi,

>I think you're right. What ProDOS does is add 3 bytes to the return
>address to pass over the command-number and command-list-address, but
>the effect is that execution continues 6 bytes after the start of the
>JSR instruction. I have read that page many times and never noticed
>that, probably because I understood what was going on with the
>return-address.

I only noticed it because I couldn't see it returning by looking for
its return point when adding 3 to the address where it was called :)
--
Colin
https://www.colino.net/

[Oliver Schmidt]

I - pretty strongly - disagree. What JSR does is an implementation detail.
What the documentation wants to express is that MLI returns 3 bytes
"behind" what a "bare" JSR returns to. Putting a 6 there instead of a 3 is
plainly wrong.

[Colin Leroy-Mira]

Hi,

>I - pretty strongly - disagree. What JSR does is an implementation
>detail. What the documentation wants to express is that MLI returns 3
>bytes "behind" what a "bare" JSR returns to. Putting a 6 there instead
>of a 3 is plainly wrong.

I see what you mean, but the original phrasing is confusing. It could
be "the MLI returns 3 bytes behind than a normal JSR does, to the
address of the JSR plus 6".

--
Colin
https://www.colino.net/

[Jerry Penner]

Colin Leroy-Mira <co...@colino.net> writes:

> Hi,

The P8 tech-ref states:

----------------------------------------
SYSCALL JSR MLI ;Call Command Dispatcher
DB CMDNUM ;This determines which call is being made
DW CMDLIST ;A two-byte pointer to the parameter list
BNE ERROR ;Error if nonzero

Upon completion of the call, the MLI returns to the address of the
JSR plus 3 (in the above example, the BNE statement); the call
number and parameter list pointer are skipped.
----------------------------------------

Where the CPU returns to is 6 bytes past the label SYSCALL, which is the
location of the "BNE ERROR" instruction.

I think I always read and understood the book the way I think Oliver
does, but I think the book's address arithmetic is wrong, looking at it
now.

[Jeff Blakeney]

On 2023-08-24 2:06 a.m., Jerry Penner wrote:

> Colin Leroy-Mira <co...@colino.net> writes:
>
> The P8 tech-ref states:
>
> ---------------------------------------- SYSCALL JSR MLI ;Call
> Command Dispatcher DB CMDNUM ;This determines which call is being
> made DW CMDLIST ;A two-byte pointer to the parameter list BNE
> ERROR ;Error if nonzero
>
> Upon completion of the call, the MLI returns to the address of the
> JSR plus 3 (in the above example, the BNE statement); the call number
> and parameter list pointer are skipped.
> ----------------------------------------
>
> Where the CPU returns to is 6 bytes past the label SYSCALL, which is
> the location of the "BNE ERROR" instruction.
>
> I think I always read and understood the book the way I think Oliver
> does, but I think the book's address arithmetic is wrong, looking at
> it now.

As far as I remember my 6502 assembly, when you do a JSR, the processor
has already read the JSR byte and the two byte address so the PC is
pointing at the DB CMDNUM statement and that is what is pushed on the
stack. The MLI pulls that address off the stack, adds 3 to it, pushes
it back on the stack and does an RTS so that the BNE ERROR statement
will get executed.

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.

[Michael J. Mahon]

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.

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.

Sometimes details matter. ;-)

--
-michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com

[Jeff Blakeney]

On 2023-08-28 2:45 a.m., Michael J. Mahon wrote:

> Jeff Blakeney <CUTjeffre...@yahoo.ca> wrote:
>> As far as I remember my 6502 assembly, when you do a JSR, the processor
>> has already read the JSR byte and the two byte address so the PC is
>> pointing at the DB CMDNUM statement and that is what is pushed on the
>> stack. The MLI pulls that address off the stack, adds 3 to it, pushes
>> it back on the stack and does an RTS so that the BNE ERROR statement
>> will get executed.
>>
>> 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.

I haven't done any 6502 coding in a long time but my memory tells me the
6502 did it this way: PC has address of SYSCALL (or probably SYSCALL-1
and increments the PC to get there), processor reads the byte there to
see what opcode to execute, sees it is a JSR so increments the PC and
reads the low byte of the address, increments the PC and reads the high
byte of the address, pushes the current PC (SYSCALL +2) to the stack,
changes the PC to the address it just read and continues execution from
there. When it hits an RTS it pulls the address off the stack, adds one
to it and continues executing 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.
>
> Sometimes details matter. ;-)

Breaking it down above, I realized that I probably got it wrong in my
original post and the stack gets the address of the high byte and not
the address of the DB CMDNUM. Also it makes me wonder if the 6502
subtracts one from the JSR address when it puts it in the PC so that it
can be incremented before reading the opcode at the JSR address. It
depends on whether the 6502 always increments the PC before reading the
next opcode and my memory fails me on that point.

As to the original question, the 6502 pushes SYSCALL + 2 to the stack,
the MLI needs to add 3 to the address pulled off the stack, the 6502
adds 1 when it does the RTS so you end up continuing execution at
SYSCALL +6 in the example.

Knowing the details of how the 6502 works in this case isn't really
necessary, you just need to know that your code will continue executing
after the command block. :)

[qkumba]

> In fact, "Upon completion of the call, the MLI returns to the address
> of the JSR plus 6" - not 3.

It should say to the *return address* of the JSR plus 3.

[Antoine Vignau]

EXACTLY! That is the only viable/correct/reliable/<insert positive adjective>/ sentence.

Antoine

[Peter 'Shaggy' Haywood]

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 <CUTjeffre...@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.

--


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-------------- Shaggy was here! ---------------
Ain't I'm a dawg!!

[Michael J. Mahon]

It doesn’t. Note that all the hardware vectors—IRQ, RESET, etc. are the
actual service routine addresses, not the addresses minus one.

This is in contrast with the ROM convention of “RTSing” to a routine whose
address is part of a table of “routine addresses -1” to work around the
lack of an indexed indirect JMP. (RTS takes its time to increment the
return as it pops successive bytes from the stack.)

> As to the original question, the 6502 pushes SYSCALL + 2 to the stack,
> the MLI needs to add 3 to the address pulled off the stack, the 6502
> adds 1 when it does the RTS so you end up continuing execution at
> SYSCALL +6 in the example.
>
> Knowing the details of how the 6502 works in this case isn't really
> necessary, you just need to know that your code will continue executing
> after the command block. :)

Right, but it matters a great deal to the implementor of SYSCALL, or any
other subroutine which uses in-line parameters. ;-)

[Michael J. Mahon]

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.

[Antoine Vignau]

I am a big fan of http://www.brutaldeluxe.fr/documentation/cortland/v7%2065SC816%20opcodes%20Cycle%20by%20cycle%20listing.pdf

I let you discover the opcodes, cycle by cycle listing,

Antoine

[Peter 'Shaggy' Haywood]

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.

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!"