Count your RAM banks

rje · Post by **rje** » Tue Apr 06, 2021 7:22 pm

Here's how I DID it. See what's wrong?


void determineBankCount()
{
   setRAMbank(1);
   POKE(0xa000,17); // 17 is "a very random number"
   bankCount = 256;  // starting assumption; prove me wrong.
   setRAMbank(129);
   if (PEEK(0xa000) == 17) bankCount = 128; // 1024K
   setRAMbank(65);
   if (PEEK(0xa000) == 17) bankCount = 64; // 512K
}

Here's a mo' betta' way:


void determineBankCount()
{
   bankCount = 256;

   // I could do this with a loop, but meh.

   setRAMbank(192);  
   POKE(0xb000,17);
   setRAMbank(0);
   if (PEEK(0xb000) == 17) bankCount = 192;

   setRAMbank(128);
   POKE(0xb000,17);
   setRAMbank(0);
   if (PEEK(0xb000) == 17) bankCount = 128;

   setRAMbank(64);
   POKE(0xb000,17);
   setRAMbank(0);
   if (PEEK(0xb000) == 17) bankCount = 64;
}

Lorin Millsap · Post by **Lorin Millsap** » Tue Apr 06, 2021 7:24 pm

That’s not entirely accurate. You can totally have 1536K.

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desertfish · Post by **desertfish** » Tue Apr 06, 2021 8:33 pm

If you use MEMTOP kernal routine with carry set, it will return the number of ram banks in A:


sec
jsr  MEMTOP     ; $FF99
sta  num_banks

Lorin Millsap · Post by **Lorin Millsap** » Tue Apr 06, 2021 9:49 pm

If you use MEMTOP kernal routine with carry set, it will return the number of ram banks in A:
secjsr  MEMTOP     ; $FF99sta  num_banks

Question. I’m not as much of a programmer, but is MEMTOP not mirrored in RAM?

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desertfish · Post by **desertfish** » Tue Apr 06, 2021 9:54 pm

I don't know what you mean precisely with the question sorry

Lorin Millsap · Post by **Lorin Millsap** » Tue Apr 06, 2021 9:58 pm

I don't know what you mean precisely with the question sorry

For at least most KERNAL routines they are located in RAM in a vector table. The problem with using the ROM vector table is new routines that supersede the KERNAL functions can be pointed to using the RAM vectors whereas if you use the ROM vectors you don’t get the benefit of the new routines. The commodore systems would copy the KERNAL vectors to RAM locations which meant you could inject patches. I don’t know if this routine in particular has a RAM vector or not. Sent from my iPhone using Tapatalk

desertfish · Post by **desertfish** » Tue Apr 06, 2021 10:11 pm

For what I can see, it is not mirrored via a vector in ram. The entry at $ff99 in the kernal's jump table directly jmps to another location in rom, instead of doing a vectored (indirect) jmp. Indeed a handful of kernal routines do the vectoring (not many, tbh), but MEMTOP isn't among those. Does this matter?

rje · Post by **rje** » Tue Apr 06, 2021 10:47 pm

3 hours ago, Lorin Millsap said:

That’s not entirely accurate. You can totally have 1536K.

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Ha, yeah I suspected so. Actually I do have that line in my code... but the (r38) emulator won't allow that setting for testing.


   setRAMbank(193);
   if (PEEK(0xa000) == 17) bankCount = 192;

The r38 Usage says

Quote

-ram <ramsize>

Specify banked RAM size in KB (8, 16, 32, ..., 2048).

The default is 512.

And it poops out when I run it with -ram 1536 ?

rje · Post by **rje** » Tue Apr 06, 2021 10:52 pm

2 hours ago, desertfish said:
If you use MEMTOP kernal routine with carry set, it will return the number of ram banks in A:
sec
jsr  MEMTOP     ; $FF99
sta  num_banks

You're right - I could (should) just use MEMTOP and check the accumulator. I did it brute force instead, which might actually not work in real life if the X16 doesn't do RAM mirroring.

And as far as MEMTOP goes: the KERNAL routine itself has been rejiggered to return the bank count, so I don't think it matters that the vector is in ROM.

And... correct me if I'm wrong; I've never really had to think about this, but: isn't it just the I/O KERNAL routines that COME FROM (for those of you fluent in INTERCAL) an indirect vector that's initialized into RAM at boot time? I.E. the only things you'd need to "re-vector" are I/O, isn't that correct? Oh heck, I need to go back to pagetable.com and study harder.

Wavicle · Post by **Wavicle** » Wed Apr 07, 2021 12:20 am

40 minutes ago, rje said:

You're right - I could just use MEMTOP and check the accumulator. I did it brute force instead, which might actually not work in real life if the X16 doesn't do RAM mirroring.

Your thinking isn't far off though. You can see how KERNAL does it here. The biggest difference between your implementation and KERNAL is that you use the random number 17 and KERNAL uses the random number "1 + whatever is in $a000 of bank 0". If I'm reading the code correctly, KERNAL is making the optimization assumption that the number of banks will always be a power of 2 since the loop uses the accumulator to select the bank and modifies the accumulator with asl. If loop #7 passes (testing bank #128) it calls memtop with 0 in A, which I guess means 256 banks (2 MB).

2 hours ago, desertfish said:

For what I can see, it is not mirrored via a vector in ram. The entry at $ff99 in the kernal's jump table directly jmps to another location in rom, instead of doing a vectored (indirect) jmp. Indeed a handful of kernal routines do the vectoring (not many, tbh), but MEMTOP isn't among those. Does this matter?

It is stored in the KERNAL variable area (KVAR segment, which is $0200-$02BA I think) but you'd probably need to look at the assembler map file to know where. Most importantly though: doing so would be a bad idea. There is nothing that says the location is guaranteed to be the same across all releases of KERNAL. MEMTOP is guaranteed to always return the correct value.