| ISA Flash Drive / EMS Board [message #3729] |
Sun, 12 November 2017 18:39  |
smbaker
Messages: 49
Registered: January 2017
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I had this idea while talking with folks in the micro-8088 thread, how cool it would be to have an ISA flash drive. I came up with this:
The design is basically Sergey's bank switching circuit from the Zeta-2, with four sockets. The Sockets can either be configured as 39SF040 Flash Devices or ASC6C4008 SRAM, depending on the positions of JP1 and JP2. This lets the board serve as either a bankable flash storage device, or an EMS board. I've tested EMS support using a modified version of the lo-tech EMS driver. The modification is minor -- this board disables banking on system reset until a page enable register is written, this being necessary to support the flash drive feature so we could stick a BIOS extension at the start of the flash. In addition to testing EMS, I've written a simple program to test that I can write and read when configured as a flash drive.
It supports a 64KB frame that is divided into four 16KB segments. Each segment can independently select any 16KB bank in the available memory devices. The page frame can be placed at any 64KB boundary of the x86 addressable space (E0000 being the usual place for an EMS board).
The next task is probably to write an int13 bios extension for the flash drive mode. This seems easy, especially if I chose to make it a read-only device -- just translate the C/H/S, and copy sectors, bank switching as necessary.
As to why develop a flash drive when we have more convenient devices such as XT-IDE and various compactflash boards available... why not? It reminds me of the Boca Start Card that I had back in the day.
No published schematic yet, but I'll get it put up on by blog sooner or later. There's some nits with the board, like the bracket mounting holes being uncomfortably close to the pads of the rightmost memory device.
Scott
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