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BlockIII.html
<!DOCTYPE doctype PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><title>Virtual AGC FAQ Page</title><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta name=Author content="Ronald Burkey"><link rel=icon type="image/png" href="favicon.png"><meta name=author content="Ronald S. Burkey"><script type="text/javascript" src="Header.js"></script></head><body style="background-image: url(gray3.jpg);"><script type="text/javascript">
document.write(headerTemplate.replace("@TITLE@","Block III").replace("@SUBTITLE@","Ruminations on a More-Ideal World"))
</script><br><h2><a name=Blocks_for_All_Occasions id="Blocks_for_All_Occasions"></a>Blocks
for All Occasions</h2><p>The design of the AGC went through several stages, which for the
sake of this discussion we'll call "blocks". However, the
different block designs, while similar in many ways, were quite
different in others. Rather than try to cover all of them on
one page, the different blocks are covered in separate pages:<br></p><ul><li><a href="NARA-SW/E-1077.pdf">"Block 0"</a> — Earliest
preliminary design of the AGC for which I have any
documentation, not used in any missions, and with no known
existing code. There's a single document (that I know of)
describing it, so the link is simply to that document, rather
than to a separate page.</li><li><a href="Block1.html">"Block 1"</a> — Used for the earlier
designs of the Command Module, which were also referred to a
Block 1 spacecraft. Used for the unmanned Apollo 4 and
Apollo 6 unmanned missions.</li><li><a href="assembly_language_manual.html">"Block 2"</a> — Used
for the later designs of the Command Module, which were also
referred to a Block 2 spacecraft. Used for the (unmanned)
Apollo 5 mission's Lunar Module, and for all manned missions.</li><li>"Block 3" — There was no such thing in the Apollo program, but
Hugh Blair-Smith has given us some ideas about what it might
have looked like if there were.</li></ul><p>However, <i>this</i> page confines itself just to the Block 3
design.<br></p><hr style="width: 100%; height: 2px;"><h2><a name=Intro id="Intro"></a><big><big><big><font size="-1"><big><big><big>Introduction</big></big></big></font></big></big></big></h2><p>The first thing to understand about the Block III AGC is that
there's no such thing. However, if you have had the patience
to read much of what we have to say on this site about the
architecture of the Block I AGC (used only in a handful of
unmanned missions) and especially the Block II AGC (used in all
the manned missions), you will see that certain aspects of the
architecture seem to have led to related inefficiencies in the
software, and that the system could perhaps have had a
more-efficient architecture from the ground up. <br></p><p>Of course, in blithely stating that, we shouldn't pretend that
that fact should have been obvious in advance, nor that there
weren't inescapable forces that led to this kind of
situation. For one thing, the "feature creep" (over which
the developers had no control) on the original development
project, combined with the need to maintain a certain backward
compatibility in order to keep from having to rewrite the software
over and over again, combined too with the inherent multiplicative
complexity in increasing instruction and memory word-lengths at
the time, would have forced the team to allow only incremental
changes to the architecture.<br></p><p>However, that doesn't stop us from speculating about what a
better Block III system <i>might</i> have looked like, if the
developers were freed from these kinds of restrictions. But
as it happens, we don't need to speculate about it! As he
explains in a footnote in his book, <a
href="https://www.amazon.com/Left-Brains-Right-Stuff-Computers/dp/0996434534">
Left Brains for the Right Stuff</a>, original AGC developer Hugh
Blair-Smith put together some notes for a hypothetical Block III
AGC architecture that he hoped would have doubled the processing
speed, and (to quote his words)<br></p><blockquote><p><small>"I did the thinking and writing just a few years ago,
but strictly in terms of the mid-'60s technology we actually
used. Probably the greatest risk of non-fidelity is in power
consumption, because I take advantage of the Harvard
architecture to cycle both fixed and erasable memories
simultaneously whenever practical (which is mostly)."</small><br></p></blockquote><p>And,<br></p><blockquote><p><small>"I plan to publish this fantasy on-line for the
amusement of all the people around the world who enjoy
building hardware or software replicas of the AGC".</small><br></p></blockquote><p>Actually, only the second quote is from the book. While I
don't know that "enjoy" is the word I would necessarily use for
this activity, Hugh has in fact sent his notes on the topic to us,
and we present them here for your delectation.<br></p><h2>Hugh's Dream</h2><p>I quote:<br></p><blockquote><p><small>"What I'd love is for some too-much-time-on-their-hands
people to port, say, Luminary 099 from Block II to Block III
and see how much shorter and faster it is!"</small><br></p></blockquote>
A fascinating idea, obviously. One might wish for not merely a
port of one or more of the Block II AGC programs, but perhaps even a
Block III assembler and simulator. If you're such a person,
give it some thought. Perhaps it would be your road to fame!<br><h2><a name=HughsNotes></a>Hugh's Notes<br></h2><p>Hugh's current thoughts on the improved architecture, upon which
he has been working with Bruno Muller, comprise two Word
documents:<br></p><ul><li><a href="Documents/AGC%20Block%20III%20Overview.docx">AGC
Block III Overview.docx</a></li><li><a
href="Documents/AGC%20Block%20III%20Native-Mode%20Instructions.docx">AGC
Block III Native-Mode Instructions.docx</a></li></ul><p>We also have Hugh's <i>original</i> solo thoughts on the
subject, which I believe have been completely supplanted by the
documents listed above. Nevertheless, those original
thoughts are preserved for posterity below, though <strike>struck
out</strike> to avoid confusion.<br></p><ul><li><a href="Documents/AGC_Block_III_upd_3.doc"> <strike>AGC
Block III.doc</strike></a><strike><a
href="Documents/AGC_Block_III_upd_2.doc"><br></a></strike></li><strike> </strike><li><strike><a href="Documents/AGC_Block_III_testing.doc"> AGC
Block III testing.doc</a></strike></li><strike> </strike></ul><strike> </strike><p><strike>and since they haven't exactly been put into a finalized
form, he has also taken the trouble to describe the best way of
reading them. I quote, though a brief summary might be,
"Don't start with the testing document":</strike><strike><br></strike></p><strike> </strike><blockquote><strike> </strike><p><strike><small>"... you'll probably find the testing document
pretty opaque, since it records the output of a simulator
that existed only in my headbone. In reading the doc, I
would recommend starting with the general discussions at the
beginning, then skipping over the NI (Native Interpreter)
code examples in the middle initially, and going to
the"regular" interpreter code examples toward the end—the
idea being to get your head around how Block III performs
the same kind of interpreter as Block II does. Then dive
bravely into the NI part to see how it interprets the
two-word format in RAM of what would otherwise have been
one-word native instructions in ROM. If you were a
professor, you could assign a hapless grad student to do
that! The main reason I spent so much design effort on the
NI was to make sure the native design could handle it
without too much ugliness, and I did make several
enhancements to the native design as a result.<br><br>
"For relatively short pieces of code, like those in the
document, it should be possible to have a fairly
satisfactory experience just writing source code and
eyeballing it for running time in MCT (as well as for what I
might call Nortoning)."</small></strike><br></p></blockquote><hr style="width: 100%; height: 2px;"><center> <br><span style="color: rgb(84, 89, 93); font-family: sans-serif;
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