Skip to navigation

Elite on the BBC Micro

Keyboard: DKS4 [6502SP version, I/O processor]

Name: DKS4 [Show more] Type: Macro Category: Keyboard Summary: Scan the keyboard to see if a specific key is being pressed Deep dive: The key logger
Context: See this macro in context in the source code Variations: See code variations for this macro in the different versions References: No direct references to this macro in this source file

Arguments: A The internal number of the key to check (see p.142 of the Advanced User Guide for a list of internal key numbers) Returns: A If the key in A is being pressed, A contains the original argument A, but with bit 7 set (i.e. A + 128). If the key in A is not being pressed, the value in A is unchanged
MACRO DKS4 LDX #3 \ Set X to 3, so it's ready to send to SHEILA once \ interrupts have been disabled SEI \ Disable interrupts so we can scan the keyboard \ without being hijacked STX VIA+&40 \ Set 6522 System VIA output register ORB (SHEILA &40) \ to %00000011 to stop auto scan of keyboard LDX #%01111111 \ Set 6522 System VIA data direction register DDRA STX VIA+&43 \ (SHEILA &43) to %01111111. This sets the A registers \ (IRA and ORA) so that: \ \ * Bits 0-6 of ORA will be sent to the keyboard \ \ * Bit 7 of IRA will be read from the keyboard STA VIA+&4F \ Set 6522 System VIA output register ORA (SHEILA &4F) \ to X, the key we want to scan for; bits 0-6 will be \ sent to the keyboard, of which bits 0-3 determine the \ keyboard column, and bits 4-6 the keyboard row LDA VIA+&4F \ Read 6522 System VIA output register IRA (SHEILA &4F) \ into A; bit 7 is the only bit that will have changed. \ If the key is pressed, then bit 7 will be set, \ otherwise it will be clear LDX #%00001011 \ Set 6522 System VIA output register ORB (SHEILA &40) STX VIA+&40 \ to %00001011 to restart auto scan of keyboard CLI \ Allow interrupts again ENDMACRO