Version analysis of DOT - Elite on the 6502

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: DOT
       Type: Subroutine
   Category: Dashboard

Code variation 1 of 6A variation in the comments only

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

Cassette, Flight, Master, Electron
6502SP
    Summary: Draw a dash on the compass
    Summary: Implement the #DOdot command (draw a dash on the compass)



 Arguments:

Code variation 2 of 6A variation in the comments only

This variation is blank in the 6502 Second Processor version.

Cassette, Flight, Master, Electron
   COMX                 The screen pixel x-coordinate of the dash

   COMY                 The screen pixel y-coordinate of the dash

Code variation 3 of 6A variation in the comments only

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

Cassette, Flight, Master
6502SP
Electron
   COMC                 The colour and thickness of the dash:

                          * &F0 = a double-height dash in yellow/white, for when
                            the object in the compass is in front of us

                          * &FF = a single-height dash in green/cyan, for when
                            the object in the compass is behind us
   OSSC(1 0)            A parameter block as follows:

                          * Byte #2 = The screen pixel x-coordinate of the dash

                          * Byte #3 = The screen pixel x-coordinate of the dash

                          * Byte #4 = The colour of the dash
   COMC                 The thickness of the dash:

                          * &F0 = a double-height dash in white, for when the
                            object in the compass is in front of us

                          * &FF = a single-height dash in white, for when the
                            object in the compass is behind us



.DOT

Code variation 4 of 6Specific to an individual platform

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

Master
 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

Code variation 5 of 6Related to Elite's use of the Tube

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

Cassette, Flight
Master
6502SP
Electron
 LDA COMY               \ Set Y1 = COMY, the y-coordinate of the dash
 STA Y1

 LDA COMX               \ Set X1 = COMX, the x-coordinate of the dash
 STA X1

 LDA COMC               \ Set COL = COMC, the mode 5 colour byte for the dash
 STA COL
 LDA COMX               \ Set X1 = COMX, the x-coordinate of the dash
 STA X1

 LDX COMC               \ Set COL = COMC, the mode 2 colour byte for the dash
 STX COL

 LDA COMY               \ Set Y1 = COMY, the y-coordinate of the dash
 LDY #2                 \ Fetch byte #2 from the parameter block (the dash's
 LDA (OSSC),Y           \ x-coordinate) and store it in X1
 STA X1

 INY                    \ Fetch byte #3 from the parameter block (the dash's
 LDA (OSSC),Y           \ y-coordinate) and store it in X1
 STA Y1

 INY                    \ Fetch byte #3 from the parameter block (the dash's
 LDA (OSSC),Y           \ colour) and store it in COL
 STA COL
 LDA COMY               \ Set Y1 = COMY, the y-coordinate of the dash
 STA Y1

 LDA COMX               \ Set X1 = COMX, the x-coordinate of the dash
 STA X1

 LDA COMC               \ Set A = COMC, the thickness of the dash

Code variation 6 of 6Related to the screen mode

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

Cassette, Flight
Master
6502SP
Electron
 CMP #&F0               \ If COL is &F0 then the planet/station is in front of
 BNE CPIX2              \ us and we want to draw a double-height dash, so if it
                        \ isn't &F0 jump to CPIX2 to draw a single-height dash

                        \ Otherwise fall through into CPIX4 to draw a double-
                        \ height dash
 CPX #YELLOW2           \ If the colour in X is yellow, then the planet/station
 BNE P%+8               \ is behind us, so skip the following three instructions
                        \ so we only draw a single-height dash

 JSR CPIXK              \ Call CPIXK to draw a single-height dash, i.e. the top
                        \ row of a double-height dash

 LDA Y1                 \ Fetch the y-coordinate of the row we just drew and
 DEC A                  \ decrement it, ready to draw the bottom row

.DOT2

 JSR CPIXK              \ Call CPIXK to draw a single-height dash

 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
 CMP #WHITE2            \ If the dash's colour is not white, jump to CPIX2 to
 BNE CPIX2              \ draw a single-height dash in the compass, as it is
                        \ showing that the planet or station is behind us

                        \ Otherwise the dash is white, which is in front of us,
                        \ so fall through into CPIX4 to draw a double-height
                        \ dash in the compass
 CMP #&F0               \ If COMC is &F0 then the planet/station is in front of
 BNE CPIX2              \ us and we want to draw a double-height dash, so if it
                        \ isn't &F0 jump to CPIX2 to draw a single-height dash

                        \ Otherwise fall through into CPIX4 to draw a double-
                        \ height dash