A4000 ROM size

[hooverphonique]

I was looking at replacing the roms in my A4000 with a flash ram-based solution.
From what I remember from the 90's, the A4000 has/supports 512kB of kickstart rom (@$F80000), and the memory map confirms that, but the schematic shows 2*256k*16b rom (1MB). Kickstart dumps for e.g. WinUAE are also 512kB, so does anyone know why there is 1MB of mask rom in the machine?

[cdh]

So the schematic is perhaps a bit over-zealous in labeling, as the HN62402 was 128K x 16bit. However, each socket is wired to handle a 256K x 16bit device, so I believe your calculation and assumption that the A4000 schematic shows 2*256k*16b ROM = 1MB is correct. Another way to verify this is to look at the address lines coming from the bus. You see CPU A(19:2) connected to the ROM chips? That means there are 20 bits of address -- and 2^20 is 1MB.

Now for the unfortunate news. Fat Gary (which does the address decoding for the Kickstart ROMs in A3000 and A4000) does not support greater than 512KB of ROM. The following thread from AmiBay gives more insight and an enhancement to allow 1MB kickstart images:

http://www.amibay.com/showthread.php...00-1MB-ROM-Mod

Note that Commodore's 3.1 ROM image is 512KB for all models except the CD32:

https://en.wikipedia.org/wiki/Kickstart_(Amiga)

So for the Amiga 3000 and Amiga 4000, Commodore shipped two 128Kx16b 3.1 ROMs for each system.

Looking at the memory map, I know you've spotted addresses from 0xf80000 to 0xffffff. That's the 512KB block at the top of memory where a 512KB Kickstart ROM is mapped. You might have also noticed the block at 0xe00000-0xe7ffff where the bottom of a 1MB ROM image could live. Speedgeek's enhancement (link above) implements exactly this.

[hooverphonique]

Originally Posted by cdh

So the schematic is perhaps a bit over-zealous in labeling, as the HN62402 was 128K x 16bit. However, each socket is wired to handle a 256K x 16bit device, so I believe your calculation and assumption that the A4000 schematic shows 2*256k*16b ROM = 1MB is correct. Another way to verify this is to look at the address lines coming from the bus. You see CPU A(19:2) connected to the ROM chips? That means there are 20 bits of address -- and 2^20 is 1MB.

Yes, I did notice the address lines give 1MB of rom address space, I was just thrown off by the schematic indicating the HN62402 being 256k*16.

Originally Posted by cdh

Now for the unfortunate news. Fat Gary (which does the address decoding for the Kickstart ROMs in A3000 and A4000) does not support greater than 512KB of ROM. The following thread from AmiBay gives more insight and an enhancement to allow 1MB kickstart images:

http://www.amibay.com/showthread.php...00-1MB-ROM-Mod

I just noticed that I already commented on that thread a few times back in 2012