Original ROMs

[mcdermd]

Is there any source other then ebay for ROMs?

I have an upgraded 128k that I'd like to restore. Unfortunately, it has upgraded ROMs to support 800k drives (512ke?). Is there any place to get the real ROMs? How about burning my own? My brother has a PROM writer I could use.

 

[bbraun]

On a related topic, I've been considering doing a ROM mod to my 512k, which would require custom ROM chips, which you would also require if you were going to burn your own original ROMs for your 128k.

I found this thread basically doing the inverse of what you're wanting:

http://68kmla.org/forums/viewtopic.php?f=7&t=14850&p=142219&hilit=128ke#p142219

trag indicates going from a rom image to the two chips requires separating every other byte between the two. That's an easy enough program to write, I think.

H3NRY indicates a 27c512 EEPROM works with a modification, but the pins in his description confuse me. Pin 1 on a 27c512 is A15, so setting that to pin 28 (VCC) would limit the chip to half the space, which wouldn't work for the proposed problem in that thread (Plus ROMs in a 128k). The description makes a bit more sense to me, with the chip enable line, pin 20 (it could also be output enable, pin 22).

Anyway, just some related information I noticed, hopefully it helps.

 

[H3NRY]

My mistake. I meant to say 27C256, the 32KB EPROM. Two of them for 64K of ROM. Pin 1 on the 27C256 is Vpp, the programming voltage pin which needs to be at 5V for normal ROM read.

See here:

viewtopic.php?f=7&t=15556&p=148698&hilit=27C256#p148698

 

[bbraun]

Thanks. FWIW, I finally got around to playing with it, and the 27c512 works unmodified (although for 64k roms, burn it into both halves and call it good). I just wrote a trivial C program to split the rom image into the high and low images, burned the two, popped the roms in, done.

 

[trag]

Thanks. FWIW, I finally got around to playing with it, and the 27c512 works unmodified (although for 64k roms, burn it into both halves and call it good). I just wrote a trivial C program to split the rom image into the high and low images, burned the two, popped the roms in, done.

So what was the custom ROM you mentioned in your earlier post. I.e., how did you modify the ROM?

On many chip programmers, the software that comes with the programmer includes an option to interleave the source file when programming chips. Writing a C program as you did works too.

 

[bbraun]

So what was the custom ROM you mentioned in your earlier post. I.e., how did you modify the ROM?

I'm just messing around. If I do anything interesting, I'll post it.

But, going through the exercise is similar to what the OP wanted to do.

OP: PM me if you want the ROM splitting code or anything else. Seems like either a 27c256 with the pin lifted or a 27c512 with the rom image duplicated should work for you.

 

[Dennis Nedry]

If you mod a ROM, it may very well still work with an emulator, but on a real Mac, you will need to give it a new checksum or else you will get a sad mac. The checksum is stored as the first 4 bytes of the overall ROM, when it's put back together. I made a simple program that can compute a new checksum for you:

http://www.d.umn.edu/~bold0070/projects/checksum/

Run your modded ROM file through it. It will tell you that the checksum failed, and it will give you the actual checksum. Simply paste this over the first 4 bytes of the file and run it again to make sure it passes.

[edit]

On second thought, I'm not so sure that it does tell you the actual checksum! If you're not familiar with that sort of programming, let me know and I'll fix it for you.

 

[mcdermd]

So the 512k and 128k both use the same 64k ROM chips. Do they use the same actual ROM code? For instance, could you use a 512k ROM set in a 128k board or vice versa?

 

[Byrd]

Yes, I believe you can install 512K ROMS (even Plus ROMS) in a 128K, which will give you the ability to run a HD20 (serial) hard disk. There was similar discussion on here a while back about this, have a look around.

JB

 

[Mac128]

You are remembering correctly Byrd. The 128K & 512K were essentially identical except for RAM. The 512Ke was a 512K with a Plus ROM (the Plus being essentially being identical to a 128K except for more RAM and built-in SCSI hardware). A few folks on here have even put a Plus ROM in a 128K to get SCSI capabilities, as well as native support for Apple's 800K drives and HD20. The ROM compatibility breaks with the SE.

 

[mcdermd]

So the 512k ROMs have support for the HD20 while the 128k ROMs do not? That would mean the code on each is different?

 

[trag]

So the 512k ROMs have support for the HD20 while the 128k ROMs do not? That would mean the code on each is different?

Naw, the post you're looking at mis-wrote 512K where I think 512Ke was meant.

The 128K and 512K ROMs were the same and lack HD20 support.

The 512KE and Mac Plus ROMs were the same and have HD20 and SCSI support. There were a couple more revisions of the Plus ROMs, IIRC.

1) So, one could start out with an original 128K... No SCSI support, no built-in HD20 support.

2) Upgrade the RAM to 512KB to have the equivalent of a Mac 512K. ROMs stay the same.

3) Replace the ROM with the Plus/512Ke ROM and replace the 400K floppy drive with an 800K floppy drive, to convert the 512K to a 512Ke. The ROM now has SCSI support, but there's no SCSI hardware on the logic board. However, an HD20 plugged into the floppy port should work without additional software, IIRC.

4) Replace the 512K logic board, but keep the ROMs and install a Mac Plus logic board to turn the machine into a Mac Plus, but this would require a new back 'bucket' for the case, as the port holes are different.

5/6) Replace the Mac Plus ROMs a couple of times as very minor updates are made.