Skip to navigation

Elite on the BBC Micro

Version analysis of PIX

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

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


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 version to expand it, and tap it again to show to all variations.

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 version to expand it, and tap it again to show to all variations.

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 version to expand it, and tap it again to show to all variations.

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 \ \ See the deep dive on "Drawing pixels in the Electron \ version" for details LSR A \ Set A = A >> 3 LSR A \ = y div 8 LSR A \ = character row number \ 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 AND %111
 AND #%00000111
 TAY

 TXA                    \ Set X = X AND %111
 AND #%00000111
 TAX

Code variation 7 of 8Related to a standard feature

In the cassette version, 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 version to expand it, and tap it again to show to all variations.

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

RSS Feed