Begin forwarded message: From: Dave Yost Subject: Re: [TUHS] 386BSD released Date: July 16, 2021 at 9:21:53 AM PDT To: Bakul Shah Plz forward this thanks This was in early 1983 or late 1982. We got the serial driver to go 19200 out and 9600 in. I did 2 things in the Fortune Systems 68k serial driver: • hand-coded asm pseudo-DMA, suggested by Robert P Warnock III • cblock size 128 bytes instead of 8, count ’em, 8. From Lyons, https://cs3210.cc.gatech.edu/r/unix6.pdf <https://cs3210.cc.gatech.edu/r/unix6.pdf> the unix v6 serial driver used a clist of cblocks, like this: The pseudo-DMA interrupt handler was a function made up of a few hand-coded 68k instructions, entered into C code as hex data. That code transferred one byte into or out of a cblock, and at the end of the cblock it grabbed the next cblock from a queue and rang the “doorbell” hardware interrupt, which caused a “software interrupt” at lower priority for further processing. Rob put the doorbell into the architecture with a couple of gates on the board because he was well aware of this software interrupt trick, which was already used in bsd. For some reason I didn’t look at the bsd code, probably because Rob’s explanation was lucid and sufficient. I once had occasion to mention this, and specifically the relaxing of the draconian 8 byte cblock size, to Dennis Ritchie. He said, sure, why not, the 8 byte cblock size was just a neglected holdover from early days. This approach was just an interrupt version of what I had proposed to Rick Kiessig as a first project at Fortune Systems: to get a 30x speed up when writing to the Fortune Systems memory-mapped character display hardware. I had done the same thing a few years earlier in Z80 in C code in a serial CRT terminal. It’s simple and obvious: make the inner loop do as little as possible. The most primitive operation needs to be a block operation, not a byte-at-a-time operation.