Drawing the screen: B%

       Name: B%                                                      [Show more]
       Type: Variable
   Category: Drawing the screen
    Summary: VDU commands for changing to a standard mode 4 screen
    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 (Part 2 of 5) uses 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).

 The Electron version of Elite is unique in that it uses a standard mode 4
 screen, rather than the custom square mode used in the BBC versions. This is
 because the Electron lacks the 6845 CRTC chip, which the BBC versions use to
 customise the mode.

 To make the Electron screen appear square like the BBC versions, there is a
 blank 32-byte (&20-byte) margin on each end of each character row, so each
 character row consists of 32 blank bytes on the left, then a page (256 bytes)
 of screen memory containing the game display, then another 32 blank bytes on
 the right. Screen memory is from &5800 to &7FFF, and the bottom row from &7EC0
 to &7FFF is left blank, again to be consistent with look of the BBC version.
 This means the screen takes up more memory on the Electron version than on the
 BBC versions, despite showing the same amount of content.

 On top of this, the Electron also lacks the Video ULA of the BBC Micro, so the
 famous split-screen mode of the BBC versions can't be implemented in the
 Electron version, as the BBC versions reprogram the ULA to create the coloured
 dashboard. As a result, not only does the Electron suffer from the bigger
 memory footprint of the screen, it also has to stick to the same palette for
 the whole screen, so while the space view is the same monochrome mode 4 view
 as in the BBC versions, the dashboard has to be in the same screen mode, so
 it's also monochrome (though it has twice the number of horizontal pixels as
 the four-colour mode 5 dashboard of the BBC versions, so it is noticeably
 sharper, at least).

 The following are also set up:

   * The text window is 9 rows high and 15 columns wide, and is at (8, 10)

   * The cursor is disabled


.B%

 EQUB 22, 4             \ Switch to screen mode 4

IF DISC

 EQUB 28                \ Define a text window as follows:
 EQUB 8, 19, 23, 10     \
                        \   * Left = 8
                        \   * Right = 23
                        \   * Top = 10
                        \   * Bottom = 19
                        \
                        \ i.e. 9 rows high, 15 columns wide at (8, 10)

ELSE

 EQUB 28                \ Define a text window as follows:
 EQUB 8, 23, 23, 14     \
                        \   * Left = 8
                        \   * Right = 23
                        \   * Top = 14
                        \   * Bottom = 23
                        \
                        \ i.e. 9 rows high, 15 columns wide at (8, 14)
                        \
                        \ This is slightly lower than the default window, as
                        \ otherwise the cassette's loading message overwrites
                        \ the main code file as it loads into screen memory

ENDIF

 EQUB 23, 1, 0, 0       \ Disable the cursor
 EQUB 0, 0, 0
 EQUB 0, 0, 0