Version analysis of SPBLB / DOBULB - Elite on the BBC Micro

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

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

Code variations between these versions are shown below.

Code variation 1 of 6A variation in the comments only

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Cassette, Flight, Docked, Master, Electron
6502SP
       Name: SPBLB
       Name: DOBULB

       Type: Subroutine
   Category: Dashboard

Code variation 2 of 6A variation in the comments only

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Cassette, Flight, Docked, Master, Electron
6502SP
    Summary: Light up the space station indicator ("S") on the dashboard
    Summary: Implement the #DOBULB 0 command (draw the space station indicator
             bulb)

Code variation 3 of 6A variation in the comments only

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

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Master
6502SP
 Other entry points:

   BULB2                Switch main memory back into &3000-&7FFF and return from
                        the subroutine
 This routine is run when the parasite sends a #DOBULB 0 command. It draws
 (or erases) the space station indicator bulb ("S") on the dashboard.

Code variation 4 of 6Related to Elite's use of the Tube

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Cassette, Flight, Docked
Master
6502SP
Electron
.SPBLB

 LDA #24*8              \ The space station bulb is in character block number 24
                        \ with each character taking 8 bytes, so this sets the
                        \ low byte of the screen address of the character block
                        \ we want to draw to

 LDX #LO(SPBT)          \ Set (Y X) to point to the character definition in SPBT
 LDY #HI(SPBT)

                        \ Fall through into BULB to draw the space station bulb
.SPBLB

 LDA #%00001111         \ Set bits 1 and 2 of the Access Control Register at
 STA VIA+&34            \ SHEILA &34 to switch screen memory into &3000-&7FFF
.DOBULB

 TAX                    \ If the parameter to the #DOBULB command is non-zero,
 BNE ECBLB              \ i.e. this is a #DOBULB 255 command, jump to ECBLB to
                        \ draw the E.C.M. bulb instead
.SPBLB

 LDA #&20               \ Set A to the low byte of the screen address of the
                        \ space station bulb (which is at &7D20)

 LDX #LO(SPBT)          \ Set X to the low byte of the address of the character
                        \ definition in SPBT

 LDY #&7D               \ Set Y to the high byte of the screen address of the
                        \ space station bulb (which is at &7D20)

                        \ Fall through into BULB to draw the space station bulb

Code variation 5 of 6Related to an advanced feature

The advanced versions have wider "E" and "S" dashboard indicators than the original versions.

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

6502SP, Master
 LDA #16*8              \ The space station bulb is in character block number 48
 STA SC                 \ (counting from the left edge of the screen), with the
                        \ first half of the row in one page, and the second half
                        \ in another. We want to set the screen address to point
                        \ to the second part of the row, as the bulb is in that
                        \ half, so that's character block number 16 within that
                        \ second half (as the first half takes up 32 character
                        \ blocks, so given that each character block takes up 8
                        \ bytes, this sets the low byte of the screen address
                        \ of the character block we want to draw to

 LDA #&7B               \ Set the high byte of SC(1 0) to &7B, as the bulbs are
 STA SC+1               \ both in the character row from &7A00 to &7BFF, and the
                        \ space station bulb is in the right half, which is from
                        \ &7B00 to &7BFF

 LDY #15                \ Now to poke the bulb bitmap into screen memory, and
                        \ there are two character blocks' worth, each with eight
                        \ lines of one byte, so set a counter in Y for 16 bytes

.BULL

 LDA SPBT,Y             \ Fetch the Y-th byte of the bulb bitmap

 EOR (SC),Y             \ EOR the byte with the current contents of screen
                        \ memory, so drawing the bulb when it is already
                        \ on-screen will erase it

 STA (SC),Y             \ Store the Y-th byte of the bulb bitmap in screen
                        \ memory

 DEY                    \ Decrement the loop counter

 BPL BULL               \ Loop back to poke the next byte until we have done
                        \ all 16 bytes across two character blocks

Code variation 6 of 6Specific to an individual platform

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

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Master
6502SP
.BULB2

 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
 JMP PUTBACK            \ Jump to PUTBACK to restore the USOSWRCH handler and
                        \ return from the subroutine using a tail call