.DKJ1 LDA VIA+&40 \ Read 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. When the \ joystick is checked in a similar way in the TITLE \ subroutine for the "Press Fire Or Space,Commander." \ stage of the start-up screen, there's another \ unnecessary TAX instruction present, but there it's \ commented out 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 thereName: DKJ1 [View in context] Type: Subroutine Category: Keyboard Summary: Read joystick and flight controls
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).
Scan for pause, configuration and secondary flight keys
Read the joystick position
Configuration variable: VIA = &FE00
Memory-mapped space for accessing internal hardware, such as the video ULA, 6845 CRTC and 6522 VIAs (also known as SHEILA)