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Elite on the BBC Micro

Version analysis of LL30

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

Name: LL30 Type: Subroutine Category: Drawing lines

Code variation 1 of 2A variation in the comments only

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Master

6502SP

Summary: Draw a one-segment line
Summary: Draw a one-segment line by sending an OSWRCH 129 command to the I/O processor

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

Code variation 2 of 2Related to Elite's use of the Tube

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Master

6502SP

STY YSAV \ Store Y in YSAV so we can retrieve it below LDA #%00001111 \ Set bits 1 and 2 of the Access Control Register at STA VIA+&34 \ SHEILA &34 to switch screen memory into &3000-&7FFF JSR LOIN \ Draw a line from (X1, Y1) to (X2, Y2) LDA #%00001001 \ Clear bits 1 and 2 of the Access Control Register at STA VIA+&34 \ SHEILA &34 to switch main memory back into &3000-&7FFF LDY YSAV \ Retrieve the value of Y we stored above RTS \ Return from the subroutine
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
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