Dowty SBD skeleton

[AaronJackson]

The Dowty SBD card is a video output controller for the PDP-11 and VAX. Commands can be sent in ASCII or binary over serial, or through DMA transfer. The DMA interface is very simple, using three 8 bit registers to set the location of a command block in RAM. When the Ready bit is set to 1, the SBD begins execution. I'm not currently sure how it knows when it has reached the last command.

[AaronJackson]

Nice, it compiles... I will test tomorrow.

This comes after quite a few hours spent in ODT trying to get the machine to draw a string...

1000/26    ; 22 bytes total
1002/14    ; 0 FCPIX
1004/6     ; 0 FCRUB
1006/2035  ; 2  FCALP
1010/300   ; x coord
1012/300   ; y coord
1014/6     ; 6 characters
1016/62510 ; eh
1020/66154 ; ll
1022/20557 ; !o
1024/177777 ; -1

C strings are packed in byte-addressing order, so the least significant bits will become the first character - meaning we don't need to worry about swapping characters around. An odd number of characters can be terminated with a 0 - this should not be included in the character count.

Instructing the SBD to execute the command block at 1000(8) via DMA:

17760046/0
17760044/2
17760042/0