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BBC Micro Elite

Drawing lines: BEGINLIN (6502SP version)

Name: BEGINLIN [View in context] Type: Subroutine Category: Drawing lines Summary: Implement the OSWRCH 129 <size> command (start receiving a new line to draw)
The parasite asks the I/O processor to draw a line by first sending an OSWRCH 129 command to the I/O processor, to tell it to start receiving a new line to draw. That call runs this routine on the receiving I/O processor. The next parameter to this call (sent with the next OSWRCH) contains the number of bytes we are going to send containing the line's coordinates, plus 1. This routine then executes an OSWRCH 130 command, which calls the ADDBYT routine to start the I/O processor listening for more bytes from the parasite. These get added to the TABLE buffer, and when the parasite has sent all the coordinates, we draw the line. Arguments: A The number of points in the new line + 1
.BEGINLIN STA LINMAX \ Set LINMAX to the number of points in the new line + 1 LDA #0 \ Set LINTAB = 0 to point to the position of the next STA LINTAB \ free byte in the TABLE buffer (i.e. the first byte, as \ we have just reset the buffer) LDA #130 \ Execute a USOSWRCH 130 command so subsequent OSWRCH JMP USOSWRCH \ calls from the parasite can send coordinates that get \ added to TABLE, and return from the subroutine using a \ tail call