DEC VT20 boot device

Paul Birkel pbirkel at
Sun Aug 11 00:46:30 CDT 2019

>-----Original Message-----
>From: Jay Jaeger [mailto:cube1 at] 
>Sent: Saturday, August 10, 2019 11:02 PM
>To: Paul Birkel; General Discussion: On-Topic Posts
>Subject: Re: DEC VT20 boot device
>On 8/10/2019 1:56 PM, Paul Birkel via cctech wrote:
>> The DEC VT20 terminal apparently included a PDP-11/05 with a direct mapped
>> character display and was intended for text editing and typesetting.  It
>> seems to have been followed by the VT21, and then VT71/VT72, all three based
>> on an LSI-11 (KD11-F).  There's a real lack of documentation about these
>> online, although the VT72 does have a print set.
>> Apparently the VT20 used the M792-YK as its bootstrap; the Field Guide is
>> silent regarding the boot device and M792 documentation stops earlier in the
>> series of variants.
>> ...
>> So . is the boot device in these systems the remote host via the serial
>> line?  What protocol would that have been?  Something native to Typeset-11
>> and DECset-11?
>> paul 
>I wonder if, maybe, it used the same protocol as the GT40, which also
>had a boot-over-serial line capability.

That's a promising lead!  The GT40/42 User's Guide (EK-GT40-0P-002), Section 5.1 Communications Bootstrap/Read-Only Memory (ROM) describes a 256 word (GT40) and 512 word (GT42) ROM, however it appears that the bootstrap loader portion is intended to occupy 63 words which fits the M792 capacity (on the GT40 just the absolute addresses 15700-15776 (base 8)).

Section 5.1.1 Bootstrap Loader describes the packed-and-serialized 6-bit "byte" stream, including some nice diagrams.  Section 5.1.2 Character Encoding includes an illustrated example starting from a pictorialized 8-level paper tape.  Appendix D has an annotated (and unexpurgated) program listing of the full GT40 ROM, including the loader and Figure D-1 Communications Bootstrap Loader Flow Diagram.  Program comments suggest that a PDP-10 was expected as the host for a GT40.  I imagine that the same expectation would have applied for the earlier VT20?

Appendix E is similar, but for the "scrolling ROM - GT42" which appears to be a VT05 emulation  It includes more conventional loaders as well: RF11, RK11, RC11, RP11, TC11, TM11, and paper tape.  According to the program comments, "the fearsome power of the 11" is brought to bear :->.

Both loaders are credited to Jack Burness.

If I understand the listings correctly then in the smaller VT20 ROM, presumptively based on the same code, one would be expected to successfully fall off the end of the ROM into freshly loaded code that signals back to the host that a successful load has taken place.  In the GT40 with the larger ROM that acknowledgement ("SENDIT") is part of the ROM itself.


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