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Aviator on the BBC Micro

Graphics: VduPlot

Name: VduPlot [Show more] Type: Subroutine Category: Graphics Summary: Perform a plot command the standard VDU routines
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * VduMove calls VduPlot * VduPoint calls VduPlot

Arguments: A The type of plot command: * 4 = Move cursor to coordinate * 5 = Draw line from cursor to coordinate * 69 = Draw point at coordinate X The x-coordinate for the plot command (0-159) Y The y-coordinate for the plot command (0-255)
.VduPlot PHA \ Store the value of A on the stack, so we can retrieve \ it after the call to OSWRCH LDA #25 \ Start a VDU 25 command, which is the equivalent of a JSR OSWRCH \ PLOT command in BBC BASIC, and which has the following \ format: \ \ VDU 25, K, X; Y; \ \ where K is the plotting mode (i.e. move or draw) and \ X and Y are the relevant coordinates as 16-bit numbers PLA \ Retrieve the value of A that we stored on the stack JSR OSWRCH \ Write the K parameter of the VDU command, i.e. the \ type of plot command LDA #0 \ Set P = 0 STA P TXA \ Set A = X, so now (P A) contains the x-coordinate as a \ 16-bit number ASL A \ Set (P A) = (P A) * 8 ROL P \ = x-coordinate * 8 ASL A \ ROL P \ This gives the screen location of the x-coordinate in ASL A \ terms of VDU coordinates, where the screen is always ROL P \ 1280 pixels wide, which is 8 times the number of \ pixels in 160-pixel-wide mode 5 (i.e. 8 * 160 = 1280) JSR OSWRCH \ Write (P A), writing the low byte in A first LDA P \ And then the high byte in P JSR OSWRCH LDA #0 \ Set P = 0 STA P TYA \ Set A = Y, so now (P A) contains the y-coordinate as a \ 16-bit number ASL A \ Set (P A) = (P A) * 4 ROL P \ = y-coordinate * 4 ASL A \ ROL P \ This gives the screen location of the y-coordinate in \ terms of VDU coordinates, where the screen is always \ 1024 pixels high, which is 4 times the number of \ pixels in 256-pixel-high mode 5 (i.e. 4 * 256 = 1024) JSR OSWRCH \ Write (P A), writing the low byte in A first LDA P \ And then the high byte in P JSR OSWRCH RTS \ Return from the subroutine