Skip to navigation

Elite on the BBC Micro

Version analysis of DKJ1

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

Name: DKJ1 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).
.DKJ1

Code variation 1 of 2Related to an enhanced feature

The docking computer in the enhanced versions doesn't dock instantly like the cassette version, but it literally takes control of the ship and docks it for you, steering the ship into the slot by "pressing" the same keys that the player would if they were flying.

This variation is blank in the Cassette and Disc (docked) versions.

Flight, 6502SP

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

Code variation 2 of 2Related to Elite's use of the Tube

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight

Docked

6502SP

LDY #1 \ Update the key logger for key 1 in the KYTB table, so JSR DKS1 \ KY1 will be &FF if "?" (slow down) is being pressed INY \ Update the key logger for key 2 in the KYTB table, so JSR DKS1 \ KY2 will be &FF if Space (speed up) is being pressed LDA VIA+&40 \ Read 6522 System VIA input register IRB (SHEILA &40)
LDA VIA+&40 \ Read 6522 System VIA input register IRB (SHEILA &40)
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. 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 there

RSS Feed