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Elite on the BBC Micro

Keyboard: DKJ1 [6502SP version]

Name: DKJ1 [Show more] Type: Subroutine Category: Keyboard Summary: Read joystick and flight controls
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 DKJ1

Specifically, scan the keyboard for the speed up and slow down keys, and read the joystick's fire button and X and Y axes, storing the results in the key logger and the joystick position variables. This routine is only called if joysticks are enabled (JSTK = non-zero).
.DKJ1 LDA auto \ If auto is non-zero, then the docking computer is BNE auton \ currently activated, so jump to auton in DOKEY so the \ docking computer can "press" the flight keys for us LDA KTRAN+1 \ Copy the key press state for the "?" key from the STA KL+1 \ key logger buffer to the key logger LDA KTRAN+2 \ Copy the key press state for the SPACE key from the STA KL+2 \ key logger buffer to the key logger .BS1 LDA KTRAN+12 \ Fetch the key press state for the joystick 1 fire \ button from the key logger buffer, which contains \ the value of the 6522 System VIA input register IRB \ (SHEILA &40) TAX \ This instruction doesn't seem to have any effect, as \ X is overwritten in a few instructions AND #%00010000 \ Bit 4 of IRB (PB4) is clear if joystick 1's fire \ button is pressed, otherwise it is set, so AND'ing \ the value of IRB with %10000 extracts this bit EOR #%00010000 \ Flip bit 4 so that it's set if the fire button has STA KY7 \ been pressed, and store the result in the keyboard \ logger at location KY7, which is also where the A key \ (fire lasers) key is logged LDX #1 \ Call DKS2 to fetch the value of ADC channel 1 (the JSR DKS2 \ joystick X value) into (A X), and OR A with 1. This ORA #1 \ ensures that the high byte is at least 1, and then we STA JSTX \ store the result in JSTX LDX #2 \ Call DKS2 to fetch the value of ADC channel 2 (the JSR DKS2 \ joystick Y value) into (A X), and EOR A with JSTGY. EOR JSTGY \ JSTGY will be &FF if the game is configured to STA JSTY \ reverse the joystick Y channel, so this EOR does \ exactly that, and then we store the result in JSTY JMP DK4 \ We are done scanning the joystick flight controls, \ so jump to DK4 to scan for other keys, using a tail \ call so we can return from the subroutine there