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Elite on the BBC Micro

Keyboard: RDJOY (Master version)

Name: RDJOY [View in context] Type: Subroutine Category: Keyboard Summary: Read from either the analogue or digital joystick
Returns: C flag Clear if we just read from the analogue joystick, set if we just read from the digital joystick
IF _COMPACT .RDJOY LDA MOS \ If MOS = 0 then this is a Master Compact, so jump to BEQ DIGITAL \ DIGITAL to read the digital joystick rather then the \ analogue joystick, as the Compact doesn't have the \ latter CLC \ Clear the C flag to indicate that we are reading from \ the analogue joystick LDA ADCH1 \ Fetch the high byte of the joystick X value EOR JSTE \ The high byte A is now EOR'd with the value in \ location JSTE, which contains &FF if both joystick \ channels are reversed and 0 otherwise (so A now \ contains the high byte but inverted, if that's what \ the current settings say) ORA #1 \ Ensure the value is at least 1 STA JSTX \ Store the resulting joystick X value in JSTX LDA ADCH2 \ Fetch the high byte of the joystick Y value EOR #&FF \ This EOR is used in conjunction with the EOR JSTGY \ below, as having a value of 0 in JSTGY means we have \ to invert the joystick Y value, and this EOR does \ that part EOR JSTE \ The high byte A is now EOR'd with the value in \ location JSTE, which contains &FF if both joystick \ channels are reversed and 0 otherwise (so A now \ contains the high byte but inverted, if that's what \ the current settings say) EOR JSTGY \ JSTGY will be 0 if the game is configured to reverse \ the joystick Y channel, so this EOR along with the \ EOR #&FF above does exactly that STA JSTY \ Store the resulting joystick Y value in JSTY LDA VIA+&40 \ Read 6522 System VIA input register IRB (SHEILA &40) 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 BNE P%+6 \ If the joystick fire button is not being pressed, \ jump to DK4 to scan for other keys LDA #&FF \ Update the key logger at KY7 to "press" the "A" (fire) STA KY7 \ button RTS \ Return from the subroutine .DIGITAL LDX #&FF \ Set X = &FF so we can use it to "press" keys in the \ key logger LDA VIA+&60 \ Read the User 6522 VIA, which is where the Master \ Compact's digital joystick is mapped to. The pins go \ low when the joystick connection is made, so we need \ to check whether any of the following are zero: \ \ PB0 = fire \ PB1 = left \ PB2 = down \ PB3 = up \ PB4 = right LSR A \ If PB0 from the User VIA is set (fire button), skip BCS P%+4 \ the following STX KY7 \ Update the key logger at KY7 to "press" the "A" (fire) \ button LSR A \ If PB1 from the User VIA is set (left), skip the BCS P%+4 \ following STX KY3 \ Update the key logger at KY3 to "press" the "<" (roll \ left) button LSR A \ If PB2 from the User VIA is set (down), skip the BCS P%+4 \ following STX KY6 \ Update the key logger at KY6 to "press" the "S" (pitch \ forward) button \ \ Note that this is the opposite key presss to the stick \ direction, as in the default configuration, we want to \ pitch up when we pull the joystick back (i.e. when the \ stick is down). To fix this, we flip this result below LSR A \ If PB3 from the User VIA is set (up), skip the BCS P%+4 \ following STX KY5 \ Update the key logger at KY5 to "press" the "X" (pitch \ back) button \ \ Note that this is the opposite key presss to the stick \ direction, as in the default configuration, we want to \ pitch down when we push the joystick forward (i.e. \ when the stick is up). To fix this, we flip this \ result below LSR A \ If PB4 from the User VIA is set (right), skip the BCS P%+4 \ following STX KY4 \ Update the key logger at KY4 to "press" the ">" (roll \ right) button LDA JSTE \ JSTE contains &FF if both joystick channels are BEQ DIG1 \ reversed and 0 otherwise, so skip to DIG1 if the \ joystick channels are not reversed LDA KY3 \ Both the joystick channels are reversed, so swap the LDX KY4 \ values of KY3 and KY4 (to swap the roll) STX KY3 STA KY4 .DIG1 LDA JSTE \ Set A to &FF if both joystick channels are reversed \ and 0 otherwise EOR JSTGY \ JSTGY will be 0 if the game is configured to reverse \ the joystick Y channel, &FF otherwise, so A will be \ 0 if one of these is true: \ \ * Both channels are normal and Y is reversed \ * Both channels are reversed and Y is not \ \ i.e. it will be 0 if the Y channel is configured to be \ reversed BEQ DIG2 \ If the result in A is 0, skip the following to leave \ the Y channel alone, as we already set the pitch keys \ above to the opposite direction to the stick \ If we get here, then the configuration settings are \ not set to reverse the Y channel, so we now swap KY5 \ and KY6 around, as we set the pitch keys above to the \ opposite direction to the stick LDA KY5 \ The Y channel should be reversed, so swap the values LDX KY6 \ of KY5 and KY6 (to swap the pitch) STX KY5 STA KY6 .DIG2 SEC \ Set the C flag to indicate that we just read the \ digital joystick RTS \ Return from the subroutine ENDIF