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Elite on the BBC Micro

Drawing lines: LL30 [6502SP version]

Name: LL30 [Show more] Type: Subroutine Category: Drawing lines Summary: Draw a one-segment line by sending an OSWRCH 129 command to the I/O processor
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * LASLI calls LL30 * NLIN2 calls LL30 * TT15 calls LL30 * TTX66 calls LL30 * ZZAAP calls LL30

Arguments: X1 The screen x-coordinate of the start of the line Y1 The screen y-coordinate of the start of the line X2 The screen x-coordinate of the end of the line Y2 The screen y-coordinate of the end of the line
.LL30 LDA #129 \ Send an OSWRCH 129 command to the I/O processor to JSR OSWRCH \ tell it to start receiving a new line to draw. The \ parameter to this call needs to contain the number of \ bytes we are going to send for the line's coordinates, \ plus 1, which we send next LDA #5 \ Send 5 to the I/O processor as the argument to the JSR OSWRCH \ OSWRCH 129 command, so the I/O processor should expect \ 4 bytes (as we send the count plus 1) LDA X1 \ Send X1, Y1, X2 and Y2 to the I/O processor, so the JSR OSWRCH \ I/O processor will draw a line from (X1, Y1) to LDA Y1 \ (X2, Y2), returning from the subroutine using a tail JSR OSWRCH \ call LDA X2 JSR OSWRCH LDA Y2 JMP OSWRCH