21MX proms (per request

dwight dkelvey at hotmail.com
Sun Sep 13 10:10:50 CDT 2015

> From: jwest at classiccmp.org
> Dwight wrote...
> > If you still have a working machine, why not disassemble the boot ROMs.
> They are only a few instructions long.
> Not necessary, as the source and binary for all the loader roms is in the
> aforementioned manual. None of them are "only a few instructions long". Most
> of them are a couple pages of assembler code.
> > As I recall, in the boot sequence, it transfers the ROMs to ram and then
> executes it.
> Pressing the IPL/TEST button does the following:
> 1) It executes cpu diagnostic 1 (registers and a few functions) and 2 (quick
> memory test, up to 32kw) that are stored in microcode. 
> 2) It transfers any one of the four installed loader roms (as designated by
> switch register bits 15 & 14) to the last (up to 32kw) 64 words of memory.
> 3) Any I/O instructions in the loader are automatically patched during the
> transfer (based on switch register bits 11 through 6) so that the correct
> I/O address (device) is referenced.
> 4) The program counter is set to the first word of the last (up to 32kw) 64
> words.
> Then you can hit the run switch to execute the (patched) loader.
> Interesting to me... on the 2100A/S, the last 64 words of memory can be
> protected. They cannot be accessed unless the "loader enable" button/light
> has been pressed. This makes it less likely that your loader (which had to
> be hand entered on that model as it didn't support "loader roms") would
> accidentally be clobbered by other code. The loader enable button stays on
> until the computer reaches a halt instruction or the halt button is pressed.
> So the typical process was to set the program counter to the first word of
> the last 64 words (up to 32kw) of memory, press loader enable, then run.
> Once the loader finished loading whatever it's target was, it would halt
> (thus re-protecting the loader). Then the user could press run to execute
> the target code and not worry about the loader getting toasted.
> Best,
> J
 My main thinking was the best place to save a boot is to save it on paper.I recall looking at mine to see how the flag worked.( not mass storage yet so needed a serial loader ).Dwight  		 	   		  

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