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Elite on the BBC Micro

Utility routines: TTX66 [6502SP version, I/O processor]

Name: TTX66 [Show more] Type: Subroutine Category: Utility routines Summary: Clear the top part of the screen and draw a white border
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: * TT26 calls TTX66 * cls calls TTX66

Clear the top part of the screen (the space view) and draw a white border along the top and sides.
.TTX66 LDX #&40 \ Set X to point to page &40, which is the start of the \ screen memory at &4000 .BOL1 JSR ZES1 \ Call ZES1 to zero-fill the page in X, which will clear \ half a character row INX \ Increment X to point to the next page in screen \ memory CPX #&70 \ Loop back to keep clearing character rows until we BNE BOL1 \ have cleared up to &7000, which is where the dashoard \ starts .BOX LDA #%00001111 \ Set COL = %00001111 to act as a four-pixel yellow STA COL \ character byte (i.e. set the line colour to yellow) LDY #1 \ Move the text cursor to row 1 STY YC LDY #11 \ Move the text cursor to column 11 STY XC LDX #0 \ Set X1 = Y1 = Y2 = 0 STX X1 STX Y1 STX Y2 \STX QQ17 \ This instruction is commented out in the original \ source DEX \ Set X2 = 255 STX X2 JSR LOIN \ Draw a line from (X1, Y1) to (X2, Y2), so that's from \ (0, 0) to (255, 0), along the very top of the screen LDA #2 \ Set X1 = X2 = 2 STA X1 STA X2 JSR BOS2 \ Call BOS2 below, which will call BOS1 twice, and then \ fall through into BOS2 again, so we effectively do \ BOS1 four times, decrementing X1 and X2 each time \ before calling LOIN, so this whole loop-within-a-loop \ mind-bender ends up drawing these four lines: \ \ (1, 0) to (1, 191) \ (0, 0) to (0, 191) \ (255, 0) to (255, 191) \ (254, 0) to (254, 191) \ \ So that's a 2-pixel wide vertical border along the \ left edge of the upper part of the screen, and a \ 2-pixel wide vertical border along the right edge .BOS2 JSR BOS1 \ Call BOS1 below and then fall through into it, which \ ends up running BOS1 twice. This is all part of the \ loop-the-loop border-drawing mind-bender explained \ above .BOS1 LDA #0 \ Set Y1 = 0 STA Y1 LDA #2*Y-1 \ Set Y2 = 2 * #Y - 1. The constant #Y is 96, the STA Y2 \ y-coordinate of the mid-point of the space view, so \ this sets Y2 to 191, the y-coordinate of the bottom \ pixel row of the space view DEC X1 \ Decrement X1 and X2 DEC X2 JSR LOIN \ Draw a line from (X1, Y1) to (X2, Y2) LDA #%00001111 \ Set locations &4000 &41F8 to %00001111, as otherwise STA &4000 \ the top-left and top-right corners will be black (as STA &41F8 \ the lines overlap at the corners, and the EOR logic \ used by LOIN will otherwise make them black) RTS \ Return from the subroutine