Skip to navigation

Elite on the BBC Micro

Screen mode: B% [Master version, Loader]

Name: B% [Show more] Type: Variable Category: Screen mode Summary: VDU commands for setting the square mode 1 screen Deep dive: The split-screen mode Drawing monochrome pixels in mode 4
Context: See this variable in context in the source code Variations: See code variations for this variable in the different versions References: This variable is used as follows: * Elite loader calls B%

This block contains the bytes that get written by OSWRCH to set up the screen mode (this is equivalent to using the VDU statement in BASIC). It defines the whole screen using a square, monochrome mode 1 configuration; the mode 2 part for the dashboard is implemented in the IRQ1 routine. The top part of Elite's screen mode is based on mode 1 but with the following differences: * 64 columns, 31 rows (256 x 248 pixels) rather than 80, 32 * The horizontal sync position is at character 90 rather than 98, which pushes the screen to the right (which centres it as it's not as wide as the normal screen modes) * Screen memory goes from &4000 to &7EFF * In the Master version of Elite, the screen mode is actually based on mode 129 rather than mode 1, so shadow RAM (known as LYNNE) is used to store the screen memory, though in all other respects the screen mode is the same as if it were based on mode 1 * The text window is 1 row high and 13 columns wide, and is at (2, 16) * The cursor is disabled This almost-square mode 1 variant makes life a lot easier when drawing to the screen, as there are 256 pixels on each row (or, to put it in screen memory terms, there are two pages of memory per row of pixels). There is also an interrupt-driven routine that switches the bytes-per-pixel setting from that of mode 1 to that of mode 2, when the raster reaches the split between the space view and the dashboard. See the deep dive on "The split-screen mode" for details.
.B% EQUB 22, 129 \ Switch to screen mode 129 EQUB 28 \ Define a text window as follows: EQUB 2, 17, 15, 16 \ \ * Left = 2 \ * Right = 15 \ * Top = 16 \ * Bottom = 17 \ \ i.e. 1 row high, 13 columns wide at (2, 16) EQUB 23, 0, 6, 31 \ Set 6845 register R6 = 31 EQUB 0, 0, 0 \ EQUB 0, 0, 0 \ This is the "vertical displayed" register, and sets \ the number of displayed character rows to 31. For \ comparison, this value is 32 for standard modes 1 and \ 2, but we claw back the last row for storing code just \ above the end of screen memory EQUB 23, 0, 12, &08 \ Set 6845 register R12 = &08 and R13 = &00 EQUB 0, 0, 0 \ EQUB 0, 0, 0 \ This sets 6845 registers (R12 R13) = &0800 to point EQUB 23, 0, 13, &00 \ to the start of screen memory in terms of character EQUB 0, 0, 0 \ rows. There are 8 pixel lines in each character row, EQUB 0, 0, 0 \ so to get the actual address of the start of screen \ memory, we multiply by 8: \ \ &0800 * 8 = &4000 \ \ So this sets the start of screen memory to &4000 EQUB 23, 0, 1, 64 \ Set 6845 register R1 = 64 EQUB 0, 0, 0 \ EQUB 0, 0, 0 \ This is the "horizontal displayed" register, which \ defines the number of character blocks per horizontal \ character row. For comparison, this value is 80 for \ modes 1 and 2, but our custom screen is not as wide at \ only 64 character blocks across EQUB 23, 0, 2, 90 \ Set 6845 register R2 = 90 EQUB 0, 0, 0 \ EQUB 0, 0, 0 \ This is the "horizontal sync position" register, which \ defines the position of the horizontal sync pulse on \ the horizontal line in terms of character widths from \ the left-hand side of the screen. For comparison this \ is 98 for modes 1 and 2, but needs to be adjusted for \ our custom screen's width EQUB 23, 0, 10, 32 \ Set 6845 register R10 = 32 EQUB 0, 0, 0 \ EQUB 0, 0, 0 \ This is the "cursor start" register, so this sets the \ cursor start line at 0, effectively disabling the \ cursor