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ROM-Drive Project

Below is ProDOS bootable ROM-Drive from David Wilson:

Enclosed find a couple of (slightly) fuzzy jpegs of my card [front and rear shots]. As far as descriptions go, you can include this:

This card was designed about 10 years ago when I was given 4 used 27512 chips. I have used ideas from Apple's 1MB RAM card (auto increment after read) as well as some of my own. The card uses one 2716/2516 EPROM to hold a Smartport and ProDOS block device driver (current version only supports slot 7 as that was where I put it so as to be able to boot from it in preference to my 5.25" drives in slot 6) which is bank selected in the $C7xx space. During the development phase the 2716 was replaced with a 6116 SRAM chip.

The 4 27512 chips are bank selected into 4x 1 byte windows in the $C0Fx space. To read half a block (256 bytes) a page number is written to the device space and latched in an 8 bit latch. In addition, an 8 bit counter is cleared. Then 256 reads are performed to the appropriate device space location and the counter increments after each read.

In order to make best use of the limited space provided by a 64KB EPROM, the following changes were made:

1) No boot blocks in any disk image. Instead, the 2716 chip has the code from a boot block assembled into it. Advantage: 2 more blocks per disk. Disadvantage: can only boot ProDOS. Blocks 0 and 1 return all zeros when read, blocks 2..33 are stored in the 27512.

2) Disk root directory is reduced from 4 blocks to 1 (just like the slot 3 drive 2 /RAM disk). This saves another 3 blocks and only limits you to 13 files in the root directory.

The software allows any combination of 1,2,3 or 4 chip (64KB to 256KB) disk images that can fit into 4 sockets and they appear as 1 to 4 drives. As this was written before P8 v2.0.x came out I had to patch P8 to allow 4 drives in slot 7.

My next design is still on paper but involves only 3 chips - a Vantis CPLD, a Flash EEPROM and a 74LS245 bus buffer for the data bus. The Smartport code will share the Flash memory with the disk image. To write to the disk will require an application program rather than having the Smartport interface support write and format commands.

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David Wilson School of IT & CS, Uni of Wollongong, Australia david@uow.edu.au
   
All content Copyright ©1984-2003, Terence J. Boldt, unless noted otherwise.