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Elite on the BBC Micro

Dashboard: MSBAR [Elite-A, I/O processor]

Name: MSBAR [Show more] Type: Subroutine Category: Dashboard Summary: Implement the put_missle command (update a missile indicator on the dashboard)
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * tube_table calls MSBAR

This routine is run when the parasite sends a put_missle command. It updates a specified missile indicator on the dashboard to the specified colour.
.MSBAR JSR tube_get \ Get the first parameter from the parasite for the \ command: \ \ put_missle(number, colour) \ \ and store it as follows: \ \ * A = missile number ASL A \ Set missle_1 = A * 8 ASL A ASL A STA missle_1 LDA #41 \ Set SC = 41 - missle_1 SBC missle_1 \ = 40 + 1 - (A * 8) STA SC \ = 48 + 1 - ((A + 1) * 8) \ \ This is the same calculation as in the disc version's \ MSBAR routine, but because the missile number in the \ Elite-A version is in the range 0-3 rather than 1-3, \ we subtract from 41 instead of 49 to get the screen \ address \ So the low byte of SC(1 0) contains the row address \ for the rightmost missile indicator, made up as \ follows: \ \ * 48 (character block 7, as byte #7 * 8 = 48), the \ character block of the rightmost missile \ \ * 1 (so we start drawing on the second row of the \ character block) \ \ * Move right one character (8 bytes) for each count \ of A, so when A = 0 we are drawing the rightmost \ missile, for A = 1 we hop to the left by one \ character, and so on LDA #&7E \ Set the high byte of SC(1 0) to &7E, the character row STA SC+1 \ that contains the missile indicators (i.e. the bottom \ row of the screen) JSR tube_get \ Get the second parameter from the parasite for the \ command: \ \ put_missle(number, colour) \ \ and store it as follows: \ \ * A = new colour for this indicator LDY #5 \ We now want to draw this line five times to do the \ left two pixels of the indicator, so set a counter in \ Y .MBL1 STA (SC),Y \ Draw the 3-pixel row, and as we do not use EOR logic, \ this will overwrite anything that is already there \ (so drawing a black missile will delete what's there) DEY \ Decrement the counter for the next row BNE MBL1 \ Loop back to MBL1 if have more rows to draw RTS \ Return from the subroutine