21MX proms (per request

[J. David Bryan]

On Sunday, September 13, 2015 at 8:42, Jay West wrote:

> 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.

It's actually a bit more nuanced than this.  What occurs is:

 3a) Except for halt instructions, any I/O instruction referencing select
     code 10 (octal) or greater is patched by adding the select code
     value in the switch register minus 10. 

This means that (a) halt instructions, which are in the I/O group, are not 
altered, (b) DCPC I/O instructions, which reference select codes 2, 3, 6, 
or 7 are not altered, and (c) interfaces that use two select codes, e.g., 
the 7900 disc interface, get both select codes updated properly.  It also 
implies that all loaders are written to reference select code 10 (or 10 and 
11, etc.).

 3b) The two's-complement of the memory address of the first word of the
     loader is stored in the last word in memory.

This is so that loaders can check that what they are loading does not 
overwrite the loader itself.  For example, the paper tape loader does a HLT 
55 if the absolute binary tape contains a record that would overlay the 
loader executable code.

 3c) The contents of the penultimate memory location is patched by adding
     the select code value in the switch register minus 10.

This is used to patch the select code into an optional DCPC control word, 
which is a constant and not an I/O instruction and so wouldn't be patched 
by (3a).  Loaders that use DCPC, e.g., the disc loaders, place their DCPC 
control word at this location.

There's actually quite a lot going on under the hood for that IBL button 
press.

                                      -- Dave