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Elite on the BBC Micro

Utility routines: TTX66 [Master version]

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: * CHPR calls TTX66 * TT18 calls TTX66 * TTX662 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 #%00001111 \ Set bits 1 and 2 of the Access Control Register at STX VIA+&34 \ SHEILA &34 to switch screen memory into &3000-&7FFF 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 LDX #%00001111 \ Set bits 1 and 2 of the Access Control Register at STX VIA+&34 \ SHEILA &34 to switch screen memory into &3000-&7FFF LDA COL \ Store the current colour on the stack, so we can PHA \ restore it once we have drawn the border 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 STY XC \ Move the text cursor to column 1 IF _SNG47 LDX #0 \ Set X1 = Y1 = Y2 = 0 STX Y1 STX Y2 STX X1 DEX \ Set X2 = 255 STX X2 ELIF _COMPACT STZ Y1 \ Set X1 = Y1 = Y2 = 0 STZ Y2 STZ X1 LDX #255 \ Set X2 = 255 STX X2 ENDIF 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 JSR BOS2 \ call 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 LDA COL \ 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) PLA \ Restore the original colour that we stored above STA COL 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 RTS \ Return from the subroutine .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 STZ Y1 \ Set Y1 = 0 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 JMP LOIN \ Draw a line from (X1, Y1) to (X2, Y2) and return from \ the subroutine using a tail call