Count your RAM banks

Lorin Millsap · Post by **Lorin Millsap** » Wed Apr 07, 2021 6:22 pm

is it even physically possible to have a non-power-of-two number of banks? [emoji848] Having an odd number of mem chips on the board somehow seems "off" to me (but i'm a noob regarding that so yeah)

Absolutely. There is nothing about the design that limits that. In fact the base 512k is just one.

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Wavicle · Post by **Wavicle** » Wed Apr 07, 2021 6:27 pm

3 hours ago, Elektron72 said:

Yes; it appears that the current version of MEMTOP assumes that the number of banks must be a power of two.

The behavior I've predicted is actually worse: I think it will report 2048K. After testing for the existence of bank 128 (which will succeed), it will left-shift the value in A, which will cause it to contain zero upon return (side note: a zero in A from MEMTOP indicates 256 banks).

At this point, I think it's clear that the entire MEMTOP routine needs to be rewritten. Someone should open a GitHub issue about this (I might do that later today).

If I put on my computer science hat, I would point out that a binary search has the same time complexity, O(log N), as the current implementation.

If I put on my software engineer hat, I would point out that a binary search is rather a lot of easy-to-get-wrong code just for counting available memory banks.

If I put on my computer engineer hat, I would point out that there is probably a minimum increment that could be checked, probably around 64K, which would significantly reduce the size of a linear search while still getting the right number.

Having close to zero 6502 experience, I'm not the right person to fix it in any case ?

Wavicle · Post by **Wavicle** » Wed Apr 07, 2021 6:43 pm

33 minutes ago, desertfish said:

is it even physically possible to have a non-power-of-two number of banks? ? Having an odd number of mem chips on the board somehow seems "off" to me (but i'm a noob regarding that so yeah)

In general parallel bus RAM chips usually have a power of 2 memory size. I'm not 100% certain of this, but I suspect that it is probably so that there are no invalid combinations of their address wires. Off the top of my head, I cannot recall seeing such a RAM chip that was not sized as a power of 2.

Seeing a non-power (or even multiple) of 2 RAM chips on a design is reasonably common. Usually all of the memory components are the same size, but this is mostly done to save on production costs.

rje · Post by **rje** » Wed Apr 07, 2021 8:59 pm

Banked RAM consists of up to four RAM chips, each with 512K of static RAM, if I understand correctly.

Chip 1: Banks 0-63. Always present (required, since Bank 0 is used by the OS).

Chip 2: Banks 64-127.

Chip 3: Banks 128-191.

Chip 4: Banks 192-255.

Thus, a test would check one bank as a proxy for each 512K segment (say, banks 65, 129, and 193), and see if it can write/read that bank.

I wonder if a 256K static RAM would fit in those spots... horrors...

rje · Post by **rje** » Wed Apr 07, 2021 9:16 pm

Issue logged. https://github.com/commanderx16/x16-rom/issues/199

Ed Minchau · Post by **Ed Minchau** » Thu Apr 08, 2021 1:24 am

4 hours ago, rje said:

Banked RAM consists of up to four RAM chips, each with 512K of static RAM, if I understand correctly.

Chip 1: Banks 0-63. Always present (required, since Bank 0 is used by the OS).

Chip 2: Banks 64-127.

Chip 3: Banks 128-191.

Chip 4: Banks 192-255.

Thus, a test would check one bank as a proxy for each 512K segment (say, banks 65, 129, and 193), and see if it can write/read that bank.

I wonder if a 256K static RAM would fit in those spots... horrors...

Is 3 RAM chips even going to be an option? My understanding was they'd only be selling 512k, 1M, and 2M variants.

Elektron72 · Post by **Elektron72** » Thu Apr 08, 2021 1:29 am

3 minutes ago, Ed Minchau said:

Is 3 RAM chips even going to be an option? My understanding was they'd only be selling 512k, 1M, and 2M variants.

The way I interpreted the FAQ is that all Commander X16s will be sold with 512K; any additional RAM must be added by the user.

rje · Post by **rje** » Thu Apr 08, 2021 2:36 am

That's right. The X16 only comes with 512K of the banked RAM. It's socketed so it's easy to add more, and (if I recall right) they're less than $10 apiece.

Screech · Post by **Screech** » Fri May 10, 2024 7:01 pm

On the off chance someone is looking to do this in BASIC:

10 BC=0:I=0
15 BANK I
20 IF PEEK($A000)=160 THEN GOTO 40
30 BC=BC+1:I=I+1:GOTO 15
40 PRINT"TOTAL BANKS:";BC
50 PRINT"TOTAL HI-RAM:";(BC*8);"K"

It's not the most elegant way, and if you're planning on using that much ram in basic, you may want to consider assembly. But this worked for me.

ahenry3068 · Post by **ahenry3068** » Fri May 10, 2024 11:26 pm

Screech wrote: ↑Fri May 10, 2024 7:01 pm On the off chance someone is looking to do this in BASIC:
10 BC=0:I=0
15 BANK I
20 IF PEEK($A000)=160 THEN GOTO 40
30 BC=BC+1:I=I+1:GOTO 15
40 PRINT"TOTAL BANKS:";BC
50 PRINT"TOTAL HI-RAM:";(BC*8);"K"
It's not the most elegant way, and if you're planning on using that much ram in basic, you may want to consider assembly. But this worked for me.

This is a bit more elegant.

10 MT = $FF99 20 AR = $30C 30 XR = $30D 40 YR = $30E 50 PR = $30F 60 POKE PR, 1 : REM Set the Carry Flag 70 SYS MT : REM Call the Memory Address $FF99 (ROM MEMTOP) 80 BC = PEEK(AR) 90 IF BC=0 THEN BC=256 110 PRINT "TOTAL BANKS";BC 115 PRINT "HIGHEST BANK #";BC-1 120 PRINT BC*8192;" BYTES OF HIRAM"

Commander X16