80-Bus News

To configure CP/M for a 64K memory system the command MOVMODV 64 * (CR) is entered. This would generate a 64K system for the IVC card with extra reserved BIOS space to accomodate ‘SYS’. The CBIOS in CP/M would still be OLDBIOS. At this stage a copy of this 64K CP/M can be placed on to the System track by using SYSGEN, or it can be saved to disk as a named file – SAVE 43 CPM64.COM (CR). If you do this and then use a BUG program on the saved copy, it will look identical to MOVMODV.COM except that address high bytes will be changed. (It is interesting to try a CPM64 48 * command. It will try to work as if it were a MOVCPM but will probably give a SYNC ERROR.)

It is now possible to put a copy of this saved file onto a disk using the command SYSGEN CPM64.COM (CR). (Another very poorly documented feature of SYSGEN is its ability to read files off disk.) SYSGEN will read in the named file and will discard the first 800H bytes (100H-8FFH) which includes the relocation part of MOVMODV. Byte 900H will be put into the first byte of the first sector of the TRACK 0, SIDE 0 of the disk and so on up thro the CCP, BDOS and BIOS as shown in the Track Map. SYSGEN also permits one to READ the SYSTEM off one disk and WRITE it to another which is usually the most convenient way of moving the system from disk to disk.

The file CPM64.COM will be used later as the basis for the new CP/M with SYS CBIOS. It might be thought that at this stage it would only be necessary to replace the CBIOS in the file with the new one, and the job would be done. Unfortunately there is much more to do. First SYS must have all of its internal addresses correctly matched to suit a 64K System. As assembled SYS is a self relocating program with a base address of 3A00H and it is not amenable to assembly for a higher address due to the way in which it is configured. The solution to this problem is easy however. If CPM64.COM is put onto the System track and ‘BOOTED’ up, and then the required SYS is ‘run up’ in the usual way, there will automatically be a copy of the required SYS in RAM at 0EE00H, with correct addresses and workspaces initialized. This relocated SYS can be used as the new BIOS, but first it is necessary to look more closely at some other problems.

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SIMON is the ROM based program that loads the COLD BOOT LOADER that in turn loads CP/M. In my case SIMON is loaded to RAM at 0F000H from a paged EPROM card. It can be overwritten when it’s job is done, allowing use of full 64k RAM. (See Note at end.) On RESET, SIMON is loaded and one of the first jobs it does is to RESET the IVC card, after which it:–

1)	Checks to see if there is a disk in Drive ‘A’. If none is found, then SIMON outputs a ‘NO DISK’ message.
2)	If a disk is found, it checks to see that the disk can be read correctly, and if not prints a ‘READ ERROR’ message.
3)	If all is well, it reads in the first sector of Track 0 to high RAM and then copies some of it to RAM at 0000. This part is the COLD BOOT LOADER. SIMON then checks the validity of the disk by comparing the first two bytes read with an internal reference. If an error is detected the message ‘WRONG DISK’ is printed on the screen.

In the event of an error SIMON enters a Monitor mode and commands can be executed to Tabulate, Alter Memory and so on. This is extremely useful even on ‘good’ disk loads because, after loading the system and any other program of interest, the disk can be removed and RESET pressed. The contents of memory can then be examined without the disturbing effect of the usual BUG program. (The area 0F000H – 0F400H and around 0FC00H will be overwritten by SIMON)..

January–February 1983 · Volume 2 · Issue 1