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Elite on the BBC Micro

Keyboard: b_14 [Elite-A, Flight]

Name: b_14 [Show more] Type: Subroutine Category: Keyboard Summary: Scan the Delta 14b joystick buttons Deep dive: Delta 14b joystick support
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * DKS1 calls b_14

Scan the Delta 14b for the flight key given in register Y, where Y is the offset into the KYTB table above (so this is the same approach as in DKS1). The keys on the Delta 14b are laid out as follows (the top two fire buttons are treated the same as the top button in the middle row): Fire laser Fire laser Slow down Fire laser Speed up Unarm Missile Fire Missile Target missile Hyperspace Unit E.C.M. Escape pod Docking computer on In-system jump Docking computer off Arguments: Y The offset into the KYTB table of the key that we want to scan on the Delta 14b
\ --- Mod: Whole section added for Elite-A: -----------> .b_13 LDA #0 \ Set A = 0 for the second pass through the following, \ so we can check the joystick plugged into the rear \ socket of the Delta 14b adaptor .b_14 \ This is the entry point for the routine, which is \ called with A = 128 (the value of BTSK when the Delta \ 14b is enabled), and if the key we are checking has a \ corresponding button on the Delta 14b, it is run a \ second time with A = 0 TAX \ Store A in X so we can restore it below EOR b_table-1,Y \ We now EOR the value in A with the Y-th entry in BEQ b_quit \ b_table, and jump to b_quit to return from the \ subroutine if the table entry is 128 (&80) - in other \ words, we quit if Y is the offset for the roll and \ pitch controls \ If we get here, then the offset in Y points to a \ control with a corresponding button on the Delta 14b, \ and we pass through the following twice, once with a \ starting value of A = 128, and again with a starting \ value of A = 0 \ \ On the first pass, the EOR will set A to the value \ from b_table but with bit 7 set, which means we scan \ the joystick plugged into the side socket of the \ Delta 14b adaptor \ \ On the second pass, the EOR will set A to the value \ from b_table (i.e. with bit 7 clear), which means we \ scan the joystick plugged into the rear socket of the \ Delta 14b adaptor STA VIA+&60 \ Set 6522 User VIA output register ORB (SHEILA &60) to \ the value in A, which tells the Delta 14b adaptor box \ that we want to read the buttons specified in PB4 to \ PB7 (i.e. bits 4-7), as follows: \ \ On the side socket joystick (bit 7 set): \ \ %1110 = read buttons in left column (bit 4 clear) \ %1101 = read buttons in middle column (bit 5 clear) \ %1011 = read buttons in right column (bit 6 clear) \ \ On the rear socket joystick (bit 7 clear): \ \ %0110 = read buttons in left column (bit 4 clear) \ %0101 = read buttons in middle column (bit 5 clear) \ %0011 = read buttons in right column (bit 6 clear) AND #%00001111 \ We now read the 6522 User VIA to fetch PB0 to PB3 from AND VIA+&60 \ the user port (PB0 = bit 0 to PB3 = bit 3), which \ tells us whether any buttons in the specified column \ are being pressed, and if they are, in which row. The \ values read are as follows: \ \ %1111 = no button is being pressed in this column \ %1110 = button pressed in top row (bit 0 clear) \ %1101 = button pressed in second row (bit 1 clear) \ %1011 = button pressed in third row (bit 2 clear) \ %0111 = button pressed in bottom row (bit 3 clear) \ \ In other words, if a button is being pressed in the \ top row in the previously specified column, then PB0 \ (bit 0) will go low in the value we read from the user \ port BEQ b_pressed \ In the above we AND'd the result from the user port \ with the bottom four bits of the table value (the \ lower nibble). The lower nibble in b_table contains \ a 1 in the relevant position for that row that \ corresponds with the clear bit in the response from \ the user port, so if we AND the two together and get \ a zero, that means that button is being pressed, in \ which case we jump to b_pressed to update the key \ logger for that button \ \ For example, take the b_table entry for the escape pod \ button, in the right column and third row. The value \ in b_table is &34. The top nibble contains the column, \ which is &3 = %011, which means in the STA VIA+&60 \ above, we write %1011 in the first pass (when A = 128) \ to set the right column for the side socket joystick, \ and we write %0011 in the first pass (when A = 0) to \ set the right column for the rear socket joystick \ \ Now for the row. The lower nibble of the &34 value \ from b_table contains the row, so that's &4 = %0100. \ When we read the user port, then we will fetch %1011 \ from VIA+&60 if the button in the third row is being \ pressed, so when we AND the two together, we get: \ \ %0100 AND %1011 = 0 \ \ which will indicate the button is being pressed. If \ any other button is being pressed, or no buttons at \ all, then the result will be non-zero and we move on \ to the next buttton TXA \ Restore the original value of A that we stored in X BMI b_13 \ If we just did the above with A = 128, then loop back \ to b_13 to do it again with A = 0 RTS \ Return from the subroutine \ --- End of added section ---------------------------->