Version analysis of B% - Elite on the BBC Micro and NES

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (flight), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: B%
       Type: Variable
   Category: Drawing the screen

Code variation 1 of 13A variation in the comments only

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Cassette, Flight
6502SP, Master
Electron
    Summary: VDU commands for setting the square mode 4 screen
    Summary: VDU commands for setting the square mode 1 screen
    Summary: VDU commands for changing to a standard mode 4 screen

Code variation 2 of 13A variation in the comments only

This variation is blank in the Electron version.

Cassette, Flight, 6502SP, Master
  Deep dive: The split-screen mode in BBC Micro Elite
             Drawing monochrome pixels in mode 4



 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).

Code variation 3 of 13A variation in the comments only

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Cassette, Flight
6502SP, Master
Electron
 It defines the whole screen using a square, monochrome mode 4 configuration;
 the mode 5 part for the dashboard is implemented in the IRQ1 routine.

 The top part of Elite's screen mode is based on mode 4 but with the following
 differences:

   * 32 columns, 31 rows (256 x 248 pixels) rather than 40, 32

   * The horizontal sync position is at character 45 rather than 49, 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 &6000 to &7EFF, which leaves another whole page
     for code (i.e. 256 bytes) after the end of the screen. This is where the
     Python ship blueprint slots in
 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
 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:

Code variation 4 of 13A variation in the comments only

This variation is blank in the Cassette, Disc (flight), 6502 Second Processor and Electron versions.

Master
   * 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

Code variation 5 of 13A variation in the comments only

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Cassette, Flight, 6502SP, Master
Electron
   * The text window is 1 row high and 13 columns wide, and is at (2, 16)
   * The text window is 9 rows high and 15 columns wide, and is at (8, 10)


   * The cursor is disabled

Code variation 6 of 13A variation in the comments only

This variation is blank in the Electron version.

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Cassette, Flight
6502SP, Master
 This almost-square mode 4 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's one page of memory per row of pixels). For more details of the
 screen mode, see the deep dive on "Drawing monochrome pixels in mode 4".

 There is also an interrupt-driven routine that switches the bytes-per-pixel
 setting from that of mode 4 to that of mode 5, when the raster reaches the
 split between the space view and the dashboard. See the deep dive on "The
 split-screen mode" for details.
 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%

Code variation 7 of 13Related to the screen mode

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Cassette, Flight, Electron
Master
6502SP
 EQUB 22, 4             \ Switch to screen mode 4
 EQUB 22, 129           \ Switch to screen mode 129
 EQUB 22, 1             \ Switch to screen mode 1

Code variation 8 of 13Related to the screen mode

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Cassette, Flight, 6502SP, Master
Electron
 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)
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

Code variation 9 of 13A variation in the comments only

This variation is blank in the Electron version.

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Cassette, Flight
6502SP, Master
 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 4 and
                        \ 5, but we claw back the last row for storing code just
                        \ above the end of screen memory
 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

Code variation 10 of 13Related to the screen mode

This variation is blank in the Electron version.

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight
6502SP, Master
 EQUB 23, 0, 12, &0C    \ Set 6845 register R12 = &0C and R13 = &00
 EQUB 0, 0, 0           \
 EQUB 0, 0, 0           \ This sets 6845 registers (R12 R13) = &0C00 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:
                        \
                        \   &0C00 * 8 = &6000
                        \
                        \ So this sets the start of screen memory to &6000
 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

Code variation 11 of 13Related to the screen mode

This variation is blank in the Electron version.

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight
6502SP, Master
 EQUB 23, 0, 1, 32      \ Set 6845 register R1 = 32
 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 40 for
                        \ modes 4 and 5, but our custom screen is not as wide at
                        \ only 32 character blocks across

 EQUB 23, 0, 2, 45      \ Set 6845 register R2 = 45
 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 49 for modes 4 and 5, but needs to be adjusted for
                        \ our custom screen's width
 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

Code variation 12 of 13Specific to an individual platform

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Cassette, Flight, 6502SP, Master
Electron
 EQUB 23, 0, 10, 32     \ Set 6845 register R10 = %00100000 = 32
 EQUB 0, 0, 0           \
 EQUB 0, 0, 0           \ This is the "cursor start" register, and bits 5 and 6
                        \ can be used to control the appearance of the cursor:
                        \
                        \   * Bit 5 = 0 for cursor on  (for steady cursors)
                        \               for fast blink (for blinking cursors)
                        \           = 1 for cursor off (for steady cursors)
                        \               for slow blink (for blinking cursors)
                        \
                        \   * Bit 6 = 0 for a steady cursor
                        \             1 for a blinking cursor
                        \
                        \ We can therefore turn off the cursor completely by
                        \ setting it to a steady cursor (bit 6 is clear) that
                        \ is turned off (bit 5 is set)
 EQUB 23, 1, 0, 0       \ Disable the cursor
 EQUB 0, 0, 0
 EQUB 0, 0, 0

Code variation 13 of 13Specific to an individual platform

This variation is blank in the Cassette, Disc (flight), Master and Electron versions.

6502SP
 EQUB 23, 0, &87, 34    \ Set 6845 register R7 = 34
 EQUB 0, 0, 0           \
 EQUB 0, 0, 0           \ This is the "vertical sync position" register, which
                        \ defines the row number where the vertical sync pulse
                        \ is fired. This is already set to 34 for mode 1 and 2,
                        \ so I'm not sure what this VDU sequence does,
                        \ especially as the register number has bit 7 set (it's
                        \ &87 rather than 7). More investigation needed!