On 09/08/2013 04:07 PM, Julian Perry wrote: > > Having a peek at the keyboard scanning code in VICE, it seems that an opportunity may have been missed. This missed opportunity was the ability to isolate joystick activity from a keyboard scan. > Once the job of sending the column signals was taken over by the 6529 from the data buss, writing data with bits 1 and 2 set (EG $ff, or $06) to the TED keyboard latch would ensure that the joysticks could not interfere with the keyboard scan. > > The scnkey routine starts at $DB11, and the actual SCAN code is a short subroutine at $DB70. > .A contains the bit pattern for the column. .A returns bit pattern of row. > The code: > $DB70 STA $FD30 ;Write to 6529 - Column > $DB73 STA $FF08 ;Write trigger to latch TED keyboard input > $DB76 LDA $FF08 ;Read latch (Row) > $DB79 RTS ;Return > > By adding 2 bytes, simply changing it to: > > STA $FD30 ;Write to 6529 - Column > LDA #FF ;disable joystick probe > STA $FF08 ;Write trigger to latch TED keyboard input > LDA $FF08 ;Read latch (Row) > RTS ;Return > > The Joysticks could be left turned off for the duration. > > Or am I still missing the bleeding obvious somewhere??? :) No, that looks to me like it could work. I think no one really thought about it and most of the time people won't touch the joystick when they are typing so it wasn't a real problem. I assume that the code to write to the 6529 was a quick addition once that chip came into play and once it worked, no one looked at it again. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2013-09-08 15:01:07
Archive generated by hypermail 2.2.0.