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C128: Writing to $FF03 and $FF04? [message #211821] Tue, 17 December 2013 11:54 Go to next message
Harry Potter is currently offline  Harry Potter
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I am working on programming the Commodore 128 and am wondering: writing to $FF01 switches the MMU to Bank 0 and $FF02 Bank 1. My documentation says that writing to $FF03 switches to Bank 14 and $FF04 Bank 14 with RAM from Bank 1. This makes no sense, because, when I disassembled the Bank 0/1-specific access routines in the first 1k of RAM, they write to $FF01 or $FF02, perform the access, writes to $FF03 or $FF04 then exits. What do these writes *really* do?
Re: Writing to $FF03 and $FF04? [message #211909 is a reply to message #211821] Wed, 18 December 2013 01:25 Go to previous messageGo to next message
Anton Treuenfels is currently offline  Anton Treuenfels
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"Harry Potter" <rose.joseph12@yahoo.com> wrote in message
news:c3d362da-109e-453e-8cb0-41b779eec02a@googlegroups.com...
I am working on programming the Commodore 128 and am wondering: writing to
$FF01 switches the MMU to Bank 0 and $FF02 Bank 1. My documentation says
that writing to $FF03 switches to Bank 14 and $FF04 Bank 14 with RAM from
Bank 1. This makes no sense, because, when I disassembled the Bank
0/1-specific access routines in the first 1k of RAM, they write to $FF01 or
$FF02, perform the access, writes to $FF03 or $FF04 then exits. What do
these writes *really* do?

=======================

Harry, Harry, Harry -

You've really got to stop thinking that just because YOU don't understand
what's going on that it "makes no sense". The documentation IS correct, the
routines DO behave that way, and there IS a reason.

Frankly I think it would be a waste of time to try to explain it to you;
you'll have to figure it out on your own. But here's a hint: it all has to
do with how to access RAM memory from ROM routines when they both occupy the
same address space.

- Anton Treuenfels
Re: Writing to $FF03 and $FF04? [message #212037 is a reply to message #211909] Wed, 18 December 2013 15:50 Go to previous messageGo to next message
epc8 is currently offline  epc8
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On Wednesday, December 18, 2013 1:25:17 AM UTC-5, Anton Treuenfels wrote:
> "Harry Potter" <rose.joseph12@yahoo.com> wrote in message
> news:c3d362da-109e-453e-8cb0-41b779eec02a@googlegroups.com...
> I am working on programming the Commodore 128 and am wondering: writing to
> $FF01 switches the MMU to Bank 0 and $FF02 Bank 1. My documentation says
> that writing to $FF03 switches to Bank 14 and $FF04 Bank 14 with RAM from
> Bank 1. This makes no sense, because, when I disassembled the Bank
> 0/1-specific access routines in the first 1k of RAM, they write to $FF01 or
> $FF02, perform the access, writes to $FF03 or $FF04 then exits. What do
> these writes *really* do?
> =======================
> Harry, Harry, Harry -
> You've really got to stop thinking that just because YOU don't understand
> what's going on that it "makes no sense". The documentation IS correct, the
> routines DO behave that way, and there IS a reason.
> Frankly I think it would be a waste of time to try to explain it to you;
> you'll have to figure it out on your own. But here's a hint: it all has to
> do with how to access RAM memory from ROM routines when they both occupy the
> same address space.
> - Anton Treuenfels

The easiest way to handle bank switching is to call code that is located somewhere else. Suppose you are running code in RAM that wants to call a system routine in ROM. One way might be to store the target address of the ROM routine in two z-page locations. Then call a service routine on zpage. That routine uses memory mapped-I/O to switch out your RAM and switch in ROM. After jumping indirect through your two z-page locs (or the equivalent), execution returns to your service routine which disables ROM, enables RAM and then exits back to your code in RAM.

[Hint, a well known 8 bit micro has onboard ROM and 8 (or 7) expansion slots. It does this all day long for I/O and to add software based languages to the computer. The original IBM-PC did something similar. It has cassette BASIC in ROM which was later modified, and in some cases PATCHED by disk BASIC.]

--- e
Re: Writing to $FF03 and $FF04? [message #212254 is a reply to message #212037] Thu, 19 December 2013 09:12 Go to previous message
Harry Potter is currently offline  Harry Potter
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Registered: March 2012
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Senior Member
On Wednesday, December 18, 2013 3:50:43 PM UTC-5, ep...@juno.com wrote:
> The easiest way to handle bank switching is to call code that is located somewhere else. Suppose you are running code in RAM that wants to call a system routine in ROM. One way might be to store the target address of the ROM routine in two z-page locations. Then call a service routine on zpage. That routine uses memory mapped-I/O to switch out your RAM and switch in ROM. After jumping indirect through your two z-page locs (or the equivalent), execution returns to your service routine which disables ROM, enables RAM and then exits back to your code in RAM.
>
I already did the run-around issue with MemBank128/cc65: it runs your code from Bank 0 and calls routines in the first 16k of RAM to access the kernel ad I/O. Bank 1 access is achieved through routines copied to the BASIC input buffer. The question here is what Bank 14 actually *is*. I guess I should look it up myself. I should have access to the C128 PRG this weekend.
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