Skip to navigation

Elite on the BBC Micro

Keyboard: DKS4 [Electron version]

Name: DKS4 [Show more] Type: Subroutine Category: Keyboard Summary: Scan the keyboard to see if a specific key is being pressed Deep dive: The key logger
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * DOKEY calls DKS4 * RDKEY calls DKS4 * hyp calls DKS4 * RDKEY calls entry point CAPSL

Arguments: X The internal number of the key to check (see p.40 of the Electron 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 X Contains the same as A Other entry points: CAPSL Scan the keyboard to see if CAPS LOCK is being pressed
.KSCAN \ This routine is called from below, and performs the \ actual keyboard scan SEC \ Set the C flag and clear the V flag, so when we call CLV \ KEYV, it scans the keyboard just like OSBYTE 121 SEI \ Disable interrupts JMP (S%+4) \ Jump to the original value of KEYV, which is stored in \ S%+4. Because we set the C and V flags as above, this \ will scan the keyboard like OSBYTE 121, which expects \ X to be set to the internal key number to scan for, \ EOR'd with %10000000. Unlike OSBYTE 121, a direct call \ to KEYV will return negative value in both A and X if \ that key is being pressed .CAPSL LDX #&40 \ Set X to the internal key number for CAPS LOCK, and \ fall through into DKS4 to check whether it is being \ pressed .DKS4 TYA \ Store Y on the stack so we can retrieve it when we PHA \ return from the subroutine, thus preserving Y TXA \ Store the key number to check in X on the stack so PHA \ we can retrieve it below ORA #%10000000 \ Set bit 7 of the key to check for and transfer the TAX \ value to X JSR KSCAN \ Call KSCAN to check whether the key in X is being \ pressed, which returns a negative value in A and X \ if it is CLI \ Enable interrupts again (as they are disabled in \ KSCAN) TAX \ Set X to the result of the key press call above PLA \ Fetch the original argument value of X from the stack AND #%01111111 \ into A, and clear bit 7 CPX #%10000000 \ If bit 7 of the result of the key press check above is BCC P%+4 \ set, then the key in X is being pressed, so skip the \ next instruction ORA #%10000000 \ The key in X isn't being pressed, so set bit 7 of A TAX \ By this point, A contains the key number we wanted to \ check for, with bit 7 set if the key is being pressed \ and clear otherwise, which is what we want to return \ from the subroutine, but first we need to restore the \ value of Y from the stack, so we store the result A in \ X while we do that PLA \ Restore the value Y that we stored on the stack, so it TAY \ gets preserved across calls to the subroutine TXA \ And we now retrieve the result that we stored in X \ back into A, so we can return it RTS \ Return from the subroutine