[TUHS] Blit source

Paul Ruizendaal pnr at planet.nl
Thu Dec 19 10:20:18 AEST 2019

> On Dec 16, 2019, at 7:25 AM, emanuel stiebler <emu at e-bbes.com> wrote:
> On 2019-12-15 21:45, Paul Ruizendaal wrote:
>> I’m looking for source code of the original Blit as described here:
>> http://doc.cat-v.org/bell_labs/blit/blit.pdf
> Thanks for trying again. It pops up on this list every few years, but
> still no schematics (2002, 2012) ...
> Cheers

Have you seen the 5620 schematics on bitsavers?

http://bitsavers.org/pdf/att/5620/schematic/5620_logic.pdf <http://bitsavers.org/pdf/att/5620/schematic/5620_logic.pdf>

Of course it is not the Blit schematics, but it looks like a close derivative. When comparing the Hardware & Software Tradeoffs paper with the schematics and the theory of operation notes at the back, it would seem to me that much of it is (near) identical to the 68K Blit.

- The video timing circuit was probably identical (also see the figures at the back with exact timing specifications).
- The memory grid was probably (near) identical - maybe changed slightly for the option to use 256Kx1 drams.
- The arbitration circuit may have been redesigned, but it looks like the bus arbitration of the M68K was not all that different from the Bellmac. A memory cycle takes 11 ticks of the 32.7 MHz pixel clock, or about 335ns. This is consistent with the numbers mentioned in the Blit papers (e.g. the display using about 30% of memory bandwidth, etc.).
- The mouse movement circuit appears unchanged from the Blit paper, with a two-phase motion signal counted for the first 4 bits in a PAL and the rest in a TTL counter

Some things are of course different (beyond the different CPU). The 5620 has an I/O expansion port and a bit of non-volatile memory, neither of which is mentioned in the Blit papers. The memory map is totally different and the protection for null pointer dereference appears gone.

The 2x 6850 UART appears to be replaced by a single 2681 programmable DUART. The button signals are routed through the additional parallel I/O bits that this chip provides, which also takes care of interrupt generation. According to the Blit papers there were several other versions of the Blit before the final design was arrived upon. Maybe earlier designs used a 2681 DUART as well, or its close cousin SCN68681. Maybe the earlier version of button.c could work with both. If this is true, it would stand to reason that the various features of the 2681/68681 were replicated with the two 6850’s and some supporting circuitry. This hypothesis seems to fit with some of aiju’s observations in the “mmap” information file included with the Blit emulator (e.g. accessing register 25 and 27, the timer/counter and its use for sound generation).

The Blit promotional video that AT&T put on youtube in 2012 has a brief shot (at 0:45) of the logic board. This appears to show two 24 pin packages in the bottom center of the board, which are in all likelihood the 6850’s. There is no 40 pin package (i.e. no 2681/68681 chip) on that board.

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