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Zibri Account closed
Registered: May 2020 Posts: 304 |
VIA 6522 latching still unemulated.
let's imagine that on disk are recorded 5 bytes:
we call them x1 x2 x3 x4 and x5
now imagine we positioned in front of them, we skip 3 and read the fourth.
if to do that you use this code:
CLV
LDA $1C01 ; A here contains X1
LDY #$03
loop:
BVC loop
CLV
DEY
BNE loop
LDA $1C01 ; A = X1 on RH and X5 on emulators
on vice, pi1541 and ultimate64/u2+
A will contain "x5".
But on real hardware it will contain X1.
The following code instead will work on both:
CLV
LDA $1C01 ; A here contains X1
LDY #$03
loop:
BVC loop
CLV
DEY
BNE loop
LDA $1C0F ; A = X5
That happens because 1C01 keeps the last byte READ (with any LOAD operation or any other cpu instruction that does a READ) while 1C0F contains the actual shift register.
Note:
also putting a "useless" LDA $1C01 inside the loop and the reading 1C01 will work too. But still I think that's an important feature present on BOTH VIA CHIPS that should be emulated. |
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Krill
Registered: Apr 2002 Posts: 2800 |
You have just created a duplicate of one of the oldest open tickets.
https://sourceforge.net/p/vice-emu/bugs/582/
Now, this is one of the issues where something works in the emulator but doesn't work on the real thing.
This is not as bad, by a long shot, as something working on the real thing but not in the emulator.
If somebody creates software that doesn't work on the real thing but works in an emulator, joke's on them. =) |
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Zibri Account closed
Registered: May 2020 Posts: 304 |
I agree with you at a logical level but still since emulators EMULATE real hardware, they should emulate as much as they can and this is very easy to fix.
Also, without fixing this it will be trivial to include an emulator detection with a very few lines of code.
On a more important note, all emulator based on vice 1541 code (including pi1541) have a bad handling of zeroes...
With a simple program that writes bytes with mopre than 3 zeroes in them, you can easily spot the difference between an ALPS, a newtronics and an emulator.
Ultimate64 and u2+ have their own implementation and on this they are the best I have seen so far.
The test results on those look very similar to a newtronics 1541.
If u2+/u64 would also implement RPM and wobbling it would be almost perfect. |
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chatGPZ
Registered: Dec 2001 Posts: 11073 |
Quote:this is very easy to fix
should be easy to provide a patch then |
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Zibri Account closed
Registered: May 2020 Posts: 304 |
Quoting Zibri
The following code instead will work on both:
CLV
LDA $1C01 ; A here contains X1
LDY #$03
loop:
BVC loop
CLV
DEY
BNE loop
LDA $1C0F ; A = X5
Update:
this code works also ONLY in emulators.
The only code that works on RH is this:
CLV
LDA $1C01 ; A here contains X1
LDY #$03
loop:
BVC loop
CLV
LDA $1C01
DEY
BNE loop
LDA $1C01 ; A = X5
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Zibri Account closed
Registered: May 2020 Posts: 304 |
Quoting GroepazQuote:this is very easy to fix
should be easy to provide a patch then
Yes, why don't you, then? |
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chatGPZ
Registered: Dec 2001 Posts: 11073 |
You are the one who said "this is very easy to fix" - so that question goes right back to you. |
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Frantic
Registered: Mar 2003 Posts: 1625 |
You guys |
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Krill
Registered: Apr 2002 Posts: 2800 |
As Groepaz implies, there is a reason why two of the most popular emulators don't do this right. (Does Z64K, btw.? :D)
While it seems easy to implement that latch from a high-level description, there may be various subtle timings and corner cases to observe. Stuff that could potentially break many things, and thus requires extensive testing and even then might suffer regressions temporarily.
So again, i can see why things that work in a given emulator but don't work on original hardware are not exactly at the top of to-do lists, contrary to the other way around. |
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chatGPZ
Registered: Dec 2001 Posts: 11073 |
Indeed. The first step would be to create an extensive set of testcases (an not just a bunch of code snippets, but programs that actually work in the testbench). |
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Zibri Account closed
Registered: May 2020 Posts: 304 |
I also found a discrepancy in how a varying series of zeroes is handled by real hardware and emulators...
Writing on a disk for example: FF FF FF 88 84 82 80 00 FF FF FF and then reading it back gives very identifiable results.
The best results I have seen come from U2+/Ultimate64 (which only misses wobbling emulation and speed setting to be perfect)
Vice and pi1541 give very identifiable results.
Moreover if you write that series of bits at "density" 3 (32 cycles per byte) and read it back at density 1 (28 cycles per byte) the result is even more different form a real hardware in good condition. |
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