Why does Z80 RC2014 ROM image 88 not have CP/M or BASIC boot options?

[Nick Bolton]

Hi all,

Re-posting here for visibility:

https://retrocomputing.stackexchange.com/questions/26056/why-does-z80-rc2014-rom-image-88-not-have-cp-m-or-basic-boot-options

Summary: When I configure the ROM card to address 010 (88 on my ROM) to run SCM with CP/M boot option, I don't actually get a CP/M boot option.

DEVICES, DIR, HELP, RESET 

*CPM 

Bad command

*cpm 

Bad command 

*BASIC 

Bad parameter 

*basic Bad parameter

Any ideas?

Thanks,

Nick

[Spencer Owen]

Hi Nick,

As "88" is a 16k image, you should only use the ROM selection jumpers for A14 and A15, which should be set to 10 respectively.

Hope this helps

Spencer

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[Nick Bolton]

Ah, amazing. Thanks Spencer. Such a simple solution: Remove the A13 jumper!

I would to understand why removing the A13 jumper solves this. I understand that it's a 16k image and therefore takes up two 8k blocks on the ROM, and I understand why I need to set the page size to 16k, but I don't understand why setting the A13 jumper causes the boot options to disappear. 

Is it because setting A13 causes only 8k to be loaded into the page? Is there some code in the SCM image that detects the image size and only shows the boot options if 16k was loaded?

[Spencer Owen]

I'm sure Steve Cousins can confirm this, but I believe that when SCM starts up, it will expect to see the appropriate bits of code for CP/M or BASIC at certain addresses.  If these aren't there then there's no point in giving you the CP/M or BASIC boot options.

Spencer

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[Justin Skists]

IIRC, there is a "filesystem" at the top of the ROM that has the
"files" for CPM and BASIC. By having A13 set, that upper 8Kbytes is a
repeat of the first 88kbytes and won't see the filesystem.

I've fallen for the 8Kbyte vs 16Kbyte ROM swapping problem many times,
and never learn! :-D

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[Steve Cousins]

Yes, that is it. 

There is a file directory at the top of the second 8k block that tells SCM what extra functions are available in the second 8k. Without this file directory being visible SCM does not show the extra functions in the help text and it doesn't recognise the extra commands.

Essentially, SCM can be expanded with executable files (commands) and text files (help) as described by the file directory entries at the top of the ROM. Below is an example for a 32k version of SCM.

*m7f80
7F80:  FF FF FF FF FF FF FF FF  FF FF FF FF FF FF FF FF  ................
7F90:  FF FF FF FF FF FF FF FF  FF FF FF FF FF FF FF FF  ................
7FA0:  55 AA 43 50 4D 20 20 20  20 20 03 00 74 4D 44 00  U.CPM     ..tMD.
7FB0:  55 AA 43 50 4D 20 20 20  20 20 41 80 55 4C B4 00  U.CPM     A.UL..
7FC0:  55 AA 42 41 53 49 43 20  20 20 03 00 09 4D 6B 00  U.BASIC   ...Mk.
7FD0:  55 AA 57 42 41 53 49 43  20 20 01 00 03 30 52 1C  U.WBASIC  ...0R.
7FE0:  55 AA 42 41 53 49 43 20  20 20 01 00 00 30 55 1C  U.BASIC   ...0U.
7FF0:  55 AA 4D 6F 6E 69 74 6F  72 20 02 00 00 00 B7 23  U.Monitor .....#

This is all documented should you wish to learn more or add functions to the ROM yourself.

Steve

[Nick Bolton]

Thanks Steve, I might try that some day.

> This is all documented should you wish to learn more or add functions to the ROM yourself.