A Free RAM-Disk

by J T Nestor

Well, not really free, but if you have a GM888 8088 board and RAM in excess of 64 Kbytes, then my program will enable you to use the memory above 0FFFF as a RAM-Disk. The principle of operation is very simple. The program runs and sets up alternative entry points to SELDRV, READ, WRITE and WBOOT. The code at these alternative entry points checks to see if the drive is “M”, if so it calls the 8088 to move sectors of 128 bytes into the appropriate section of RAM. If not, execution is passed to the original code for these functions. It is an example of “Co-operating Code”, as mentioned in the GM888 manual, from which the change-over code was borrowed. Please do not look too carefully at the 8088 code, this was my first attempt at writing 8088 pseudo assembler, and I now realise that improvements could be made. However, THIS WORKS.

A copy of the CCP and BDOS is stored in RAM, and this version is loaded when “^C” is typed, reducing the time taken to perform a warm boot and eliminating for ever the message “Wrong system on this disk”.

After “GENSYS-ing” my system, I am told that the system ends at BIOS+0F0A (FF0A) and the new code is therefore assembled to reside there. If, because of a different disk system, your end is higher than that, it will be necessary to alter BIOSEND, up to a maximum of FF31. If BIOSEND is above that the the reservation of another page of workspace will be necessary. Please check the values between BIOS V3.2 (or BIOS V3.5) and ENDIF very carefully for your system. WBOOTUSE is the length of the original wboot code which reads in the CCP and BDOS from disk, DRIVE, SECTOR, TRACK and DMAADR are the stores accessed by the SELDRV, SETSEC, SETTRK and SETDMA commands respectively. DIRBUF is the directory buffer used by all the disk directories, and can be found by examining the SELDRV command. If anyone is unsure of the positions of these areas, or would like a copy of this program on disk, they are welcome to contact me via this magazine.

The CCP and BDOS live at 1000:0000 to 1000:15FF with the 8088 code at 1000:1FE6, the directory at 1000:2000 to 1000:2FFF, and the data areas contiguously up from there. It is thus possible to look at files in the RAM-disk using DDT86, but because the disk is “Standard” it can also be inspected and unerased by a disk doctor program such as DU.

I hope that the code, which is available by contacting me, is self explanatory and adequately commented. ●