Case Study - Restoring an Intel iSBC 88/25

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This is the story of my work to bring an Intel System up and running under the IBM DOS Operating System on 5.25-inch FDDs. The board set for this machine consists of three multibus boards: iSBC 88/25 CPU, iSBC 012B 512KB RAM and iSBC 208 DSDD FDC.

Once the system is operational with IBM DOS/MSDOS, I will use an SBX 350 Parallel Port Board to interface an IDE PATA Hard Disk Drive. I will add the necessary code to the BIOS to allow this drive to operate under DOS.

Bringing up IBM DOS on this hardware only requires creating the BIOS ROM to map the Intel hardware to the Operating System. This consists of creating and testing the required low-level drivers. This set of boards provides nearly the same LSI components as the original IBM PC XT (5160).

The Hardware Reference Manual (HRF) for the iSBC 88/25 I have is Intel Order Number 143825-001.
The Hardware Reference Manual (HRF) for the iSBC 512B I have is Intel Order Number 112748-001.
The Hardware Reference Manual (HRF) for the iSBC s08 I have is Intel Order Number 143078-001.

To do this I have:
1) An untested iSBC 88/25 CPU.
2) An untested iSBC 012B 512KB RAM.
3) A known good iSBC 208 DSDD FDC.
4) A known good 7-slot multibus card cage.
5) A known good ATX Power Supply.
6) A known good 5.25-inch dual FDD 360K drive set.
7) The manuals listed above.
8) No EPROMs on the iSBC 88/25.
9) Known good bootable DOS media.

I had no idea of the condition of the iSBC 88/25 or the iSBC 012B RAM boards. The SBC 88/25 boards were bought of the internet and had tags claiming they were both bad. I have removed all the configuration jumpers and reconfigured the boards to the default configuration.

Lacking a ROM for the iSBC 88/25, I needed to develope a BIOS ROM for the system. I had done this type of development for my MPX-16 in 1980. The MPX-16 had the same "problems" as the iSBC 88/25 - the LSI components were located at different addresses than the IBM XT and it used 8251s vice 8250s for the serial ports. Most of the hard work was already done. I have brought this code into the work area to build the ROM image on my PC. I will use the BIOS systems tests to test bout the SBC 88/25 and the SBC 012B boards.

Card Cage and Power Supply

I used the same card cage and power supply as I used for the Intel System 80/10 board set. The 7-slot cage as shown here.

Cross Development Tools

I need to be able to do 8088 cross development on my Windows machine. These 8088/8086 tools are discussed here. I used the early versions of Microsoft's MASM for assembling the BIOS ROMs. I ran them under DOSBOX on my 64-bit machine.

Develop a BIOS ROM for the iSBC 88/25

I needed to port the MPX-16 source code to build a BIOS ROM for the basic iSBC 88/25 board. I will use this ROM to test the functionality of the iSBC 88/25 and iSBC 012B boards.

Development of the BIOS ROM with embedded progress indication was required for testing. I did this by configuring the 8251 and 8253 to be able to dump messages on the console terminal early in the initialization code. This began an edit, assemble, and test cycle that continued until the BIOS ROM was completely operational. I am not sure how much of the system board testing of the XT BIOS I will implement.

iSBC 88/25 CPU Board [ ]

I needed to test the CPU board by itself and see if it worked. This was integrated in the edit, assemble, and test cycle used to develop the BIOS ROM.

iSBC 012B RAM Board [ ]

I needed to test the RAM board. To do this, I needed to get a BIOS program ported to the iSBC 88/25 CPU board. I can use the system RAM test to check the SBC 012B.

BIOS Development

I started with the original source code for the MPX-16 BIOS. I modified this code to match the I/O ports used by the LSI components on the board set in this machine. This original source code is located here.

iSBC 208 Floppy Disk Controller Board [1978]

IBM DOS Operating System

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Last updated: 19 July 2015