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Elite on the BBC Micro

Drawing pixels: PIXEL [Elite-A, Parasite]

Name: PIXEL [Show more] Type: Subroutine Category: Drawing pixels Summary: Draw a 1-pixel dot, 2-pixel dash or 4-pixel square by sending a draw_pixel command to the I/O processor
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * DOEXP calls PIXEL * PIXEL2 calls PIXEL * TT22 calls PIXEL

Draw a point at screen coordinate (X, A) with the point size determined by the distance in ZZ. This applies to the top part of the screen (the monochrome mode 4 portion). Arguments: X The screen x-coordinate of the point to draw A The screen y-coordinate of the point to draw ZZ The distance of the point (further away = smaller point)
.PIXEL PHA \ Store the y-coordinate on the stack LDA #&82 \ Send command &82 to the I/O processor: JSR tube_write \ \ draw_pixel(x, y, distance) \ \ which will draw a pixel at (x, y) with the size \ determined by the distance TXA \ Send the first parameter to the I/O processor: JSR tube_write \ \ * x = X PLA \ Send the second parameter to the I/O processor: JSR tube_write \ \ * y = A LDA ZZ \ Send the third parameter to the I/O processor: JMP tube_write \ \ * distance = ZZ \ \ and return from the subroutine using a tail call