On Sun, Sep 13, 2015 at 7:44 PM, J. David Bryan <jdbr...@acm.org> wrote: > 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 >
For the curious the source code for the Initial Binary Loader microcode is listed on pages D-16 and D-17 of this manual: http://www.bitsavers.org/pdf/hp/1000/1000_MEF_EngrRef/92851-90001_Mar81_1.1.pdf It is reasonably well commented, but still not completely obvious exactly what and why it is doing what it is doing without something like Dave's explanation above.
