Version analysis of PIX - 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: PIX
       Type: Subroutine
   Category: Drawing pixels
    Summary: Draw a single pixel at a specific coordinate

Code variation 1 of 8A variation in the comments only

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

Electron
  Deep dive: Drawing pixels in the Electron version



 Draw a pixel at screen coordinate (X, -A). The sign bit of A gets flipped
 before drawing, and then the routine uses the same approach as the PIXEL
 routine in the main game code, except it plots a single pixel from TWOS
 instead of a two pixel dash from TWOS2. This applies to the top part of the

Code variation 2 of 8A variation in the comments only

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

Cassette, Flight
6502SP, Master
Electron
 screen (the monochrome mode 4 space view).
 screen (the four-colour mode 1 space view).
 screen (the space view).


 See the PIXEL routine in the main game code for more details.


 Arguments:

   X                    The screen x-coordinate of the pixel to draw

   A                    The screen y-coordinate of the pixel to draw, negated

Code variation 3 of 8A variation in the comments only

This variation is blank in the Cassette, 6502 Second Processor and Master versions.

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

Flight
Electron
 Other entry points:

   out                  Contains an RTS
 Other entry points:

   PIX-1                Contains an RTS


.PIX

Code variation 4 of 8Related to the screen mode

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

Electron
 LDY #128               \ Set ZP = 128 for use in the calculation below
 STY ZP

 TAY                    \ Copy A into Y, for use later

 EOR #%10000000         \ Flip the sign of A

Code variation 5 of 8Related to the screen mode

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

Electron
 CMP #248               \ If the y-coordinate in A >= 248, then this is the
 BCS PIX-1              \ bottom row of the screen, which we want to leave blank
                        \ as it's below the bottom of the dashboard, so return
                        \ from the subroutine (as PIX-1 contains an RTS)

Code variation 6 of 8Related to the screen mode

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

Cassette
Flight
6502SP, Master
Electron
 LSR A                  \ Set ZP+1 = &60 + A >> 3
 LSR A
 LSR A
 ORA #&60
 STA ZP+1

 TXA                    \ Set ZP = (X >> 3) * 8
 EOR #%10000000
 AND #%11111000
 STA ZP
 LSR A                  \ Set A = A >> 3
 LSR A
 LSR A

 LSR CHKSM+1            \ Rotate the high byte of CHKSM+1 to the right, as part
                        \ of the copy protection

 ORA #&60               \ Set ZP+1 = &60 + A >> 3
 STA ZP+1

 TXA                    \ Set ZP = (X >> 3) * 8
 EOR #%10000000
 AND #%11111000
 STA ZP
 LSR A                  \ Set ZP+1 = &40 + 2 * (A >> 3)
 LSR A
 LSR A
 ASL A
 ORA #&40
 STA ZP+1

 TXA                    \ Set (C ZP) = (X >> 2) * 8
 EOR #%10000000         \
 AND #%11111100         \ i.e. the C flag contains bit 8 of the calculation
 ASL A
 STA ZP

 BCC P%+4               \ If the C flag is set, i.e. bit 8 of the above
 INC ZP+1               \ calculation was a 1, increment ZP+1 so that ZP(1 0)
                        \ points to the second page in this character row (i.e.
                        \ the right half of the row)
                        \ We now calculate the address of the character block
                        \ containing the pixel (x, y) and put it in ZP(1 0), as
                        \ follows:
                        \
                        \   ZP = &5800 + (y div 8 * 256) + (y div 8 * 64) + 32

 LSR A                  \ Set A = A >> 3
 LSR A                  \       = y div 8
 LSR A                  \
                        \ So A now contains the number of the character row
                        \ that will contain the pixel we want to draw

                        \ Also, as ZP = 128, we have:
                        \
                        \   (A ZP) = (A 128)
                        \          = (A * 256) + 128
                        \          = 4 * ((A * 64) + 32)
                        \          = 4 * ((char row * 64) + 32)

 STA ZP+1               \ Set ZP+1 = A, so (ZP+1 0) = A * 256
                        \                           = char row * 256

 LSR A                  \ Set (A ZP) = (A ZP) / 4
 ROR ZP                 \            = (4 * ((char row * 64) + 32)) / 4
 LSR A                  \            = char row * 64 + 32
 ROR ZP

 ADC ZP+1               \ Set ZP(1 0) = (A ZP) + (ZP+1 0) + &5800
 ADC #&58               \             = (char row * 64 + 32)
 STA ZP+1               \               + char row * 256
                        \               + &5800
                        \
                        \ which is what we want, so ZP(1 0) contains the address
                        \ of the first visible pixel on the character row
                        \ containing the point (x, y)

 TXA                    \ To get the address of the character block on this row
 EOR #%10000000         \ that contains (x, y):
 AND #%11111000         \
 ADC ZP                 \   ZP(1 0) = ZP(1 0) + (X >> 3) * 8
 STA ZP

 BCC P%+4               \ If the addition of the low bytes overflowed, increment
 INC ZP+1               \ the high byte

                        \ So ZP(1 0) now contains the address of the first pixel
                        \ in the character block containing the (x, y), taking
                        \ the screen borders into consideration

 TYA                    \ Set Y = Y mod 8, which is the pixel row within the
 AND #7                 \ character block at which we want to draw our pixel
 TAY                    \ (as each character block has 8 rows)

 TXA                    \ Set X = X mod 8, which is the horizontal pixel number
 AND #7                 \ within the character block where the pixel lies (as
 TAX                    \ each pixel line in the character block is 8 pixels
                        \ wide)

Code variation 7 of 8Related to a standard feature

In the cassette and Electron versions, the loading screen's Saturn has a much higher dot density than the other versions, as the drawing routine plots individual pixels into the screen using OR logic, so pixels within a character block can be next to each other. The other versions poke whole one-pixel bytes directly into screen memory without the OR logic, which overwrites any pixels already plotted in that byte and ensures a much greater pixel spacing (though pixels at the ends of neighbouring character blocks can still be next to each other).

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

Cassette, Electron
Flight, 6502SP, Master
 LDA TWOS,X             \ Fetch a pixel from TWOS and OR it into ZP+Y
 ORA (ZP),Y
 STA (ZP),Y
 LDA TWOS,X             \ Fetch a pixel from TWOS and poke it into ZP+Y
 STA (ZP),Y

Code variation 8 of 8A variation in the labels only

This variation is blank in the Cassette, 6502 Second Processor, Master and Electron versions.

Flight
.out

 RTS                    \ Return from the subroutine