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Elite on the BBC Micro

Version analysis of CPIX2

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

Code variations between these versions are shown below.

Name: CPIX2 Type: Subroutine Category: Drawing pixels Summary: Draw a single-height dash on the dashboard

Code variation 1 of 20A variation in the comments only

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

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Cassette, Flight, Docked

Electron


Code variation 2 of 20A variation in the comments only

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Cassette, Flight, Docked

6502SP, Master

Electron

Draw a single-height mode 5 dash (1 pixel high, 2 pixels wide).
Draw a single-height mode 2 dash (1 pixel high, 2 pixels wide).
Draw a single-height mode 4 dash (1 pixel high, 4 pixels wide).
Arguments: X1 The screen pixel x-coordinate of the dash

Code variation 3 of 20A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Electron

Master

Y1 The screen pixel y-coordinate of the dash
A The screen pixel y-coordinate of the dash

Code variation 4 of 20A variation in the comments only

This variation is blank in the Electron version.

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Cassette, Flight, Docked

6502SP, Master

COL The colour of the dash as a mode 5 character row byte
COL The colour of the dash as a mode 2 character row byte

Code variation 5 of 20A variation in the comments only

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

Master

Returns: R The dash's right pixel byte
.CPIX2

Code variation 6 of 20Related to the screen mode

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

Electron

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

Code variation 7 of 20Specific to an individual platform

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Cassette, Flight, Docked, 6502SP, Electron

Master

LDA Y1 \ Fetch the y-coordinate into A
STA Y1 \ Store the y-coordinate in Y1

Code variation 8 of 20A variation in the comments only

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

Cassette

\.CPIX \ This label is commented out in the original source. It \ would provide a new entry point with A specifying the \ y-coordinate instead of Y1, but it isn't used anywhere

Code variation 9 of 20Related to the screen mode

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master

TAY \ Store the y-coordinate in Y

Code variation 10 of 20Related to the screen mode

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Cassette, Flight, Docked

6502SP, Master

Electron

LSR A \ Set A = A / 8, so A now contains the character row we LSR A \ need to draw in (as each character row contains 8 LSR A \ pixel rows) ORA #&60 \ Each character row in Elite's screen mode takes up one \ page in memory (256 bytes), so we now OR with &60 to \ get the page containing the dash (see the comments in \ routine TT26 for more discussion about calculating \ screen memory addresses) STA SCH \ Store the screen page in the high byte of SC(1 0) LDA X1 \ Each character block contains 8 pixel rows, so to get AND #%11111000 \ the address of the first byte in the character block \ that we need to draw into, as an offset from the start \ of the row, we clear bits 0-2 STA SC \ Store the address of the character block in the low \ byte of SC(1 0), so now SC(1 0) points to the \ character block we need to draw into
LDA ylookup,Y \ Look up the page number of the character row that STA SC+1 \ contains the pixel with the y-coordinate in Y, and \ store it in the high byte of SC(1 0) at SC+1 LDA X1 \ Each character block contains 8 pixel rows, so to get AND #%11111100 \ the address of the first byte in the character block ASL A \ that we need to draw into, as an offset from the start \ of the row, we clear bits 0-1 and shift left to double \ it (as each character row contains two pages of bytes, \ or 512 bytes, which cover 256 pixels). This also \ shifts bit 7 of X1 into the C flag STA SC \ Store the address of the character block in the low \ byte of SC(1 0), so now SC(1 0) points to the \ character block we need to draw into BCC P%+5 \ If the C flag is clear then skip the next two \ instructions INC SC+1 \ The C flag is set, which means bit 7 of X1 was set \ before the ASL above, so the x-coordinate is in the \ right half of the screen (i.e. in the range 128-255). \ Each row takes up two pages in memory, so the right \ half is in the second page but SC+1 contains the value \ we looked up from ylookup, which is the page number of \ the first memory page for the row... so we need to \ increment SC+1 to point to the correct page CLC \ Clear the C flag
\ We now calculate the address of the character block \ containing the pixel (x, y) and put it in SC(1 0), as \ follows: \ \ SC = &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 SC = 128, we have: \ \ (A SC) = (A 128) \ = (A * 256) + 128 \ = 4 * ((A * 64) + 32) \ = 4 * ((char row * 64) + 32) STA SC+1 \ Set SC+1 = A, so (SC+1 0) = A * 256 \ = char row * 256 LSR A \ Set (A SC) = (A SC) / 4 ROR SC \ = (4 * ((char row * 64) + 32)) / 4 LSR A \ = char row * 64 + 32 ROR SC ADC SC+1 \ Set SC(1 0) = (A SC) + (SC+1 0) + &5800 ADC #&58 \ = (char row * 64 + 32) STA SC+1 \ + char row * 256 \ + &5800 \ \ which is what we want, so SC(1 0) contains the address \ of the first visible pixel on the character row \ containing the point (x, y) LDA X1 \ Each character block contains 8 pixel rows, so to get AND #%11111000 \ the address of the first byte in the character block \ that we need to draw into, as an offset from the start \ of the row, we clear bits 0-2 ADC SC \ And add the result to SC(1 0) to get the character STA SC \ block on the row we want BCC P%+4 \ If the addition of the low bytes overflowed, increment INC SC+1 \ the high byte \ So SC(1 0) now contains the address of the first pixel \ in the character block containing the (x, y), taking \ the screen borders into consideration

Code variation 11 of 20Related to the screen mode

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Cassette, Flight, Docked, 6502SP, Master

Electron

TYA \ Set Y to just bits 0-2 of the y-coordinate, which will AND #%00000111 \ be the number of the pixel row we need to draw into TAY \ within the character block
LDA Y1 \ Set Y to just bits 0-2 of the y-coordinate, which will AND #%00000111 \ be the number of the pixel row we need to draw into TAY \ within the character block

Code variation 12 of 20Related to the screen mode

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Cassette, Flight, Docked

6502SP, Master

Electron

LDA X1 \ Copy bits 0-1 of X1 to bits 1-2 of X, and clear the C AND #%00000110 \ flag in the process (using the LSR). X will now be LSR A \ a value between 0 and 3, and will be the pixel number TAX \ in the character row for the left pixel in the dash. \ This is because each character row is one byte that \ contains 4 pixels, but covers 8 screen coordinates, so \ this effectively does the division by 2 that we need LDA CTWOS,X \ Fetch a mode 5 1-pixel byte with the pixel position AND COL \ at X, and AND with the colour byte so that pixel takes \ on the colour we want to draw (i.e. A is acting as a \ mask on the colour byte)
LDA X1 \ Copy bit 1 of X1 to bit 1 of X. X will now be either AND #%00000010 \ 0 or 2, and will be double the pixel number in the TAX \ character row for the left pixel in the dash (so 0 \ means the left pixel in the 2-pixel character row, \ while 2 means the right pixel) LDA CTWOS,X \ Fetch a mode 2 1-pixel byte with the pixel position AND COL \ at X/2, and AND with the colour byte so that pixel \ takes on the colour we want to draw (i.e. A is acting \ as a mask on the colour byte)
LDA X1 \ Set X to just bits 0-2 of the x-coordinate, which will AND #%00000111 \ be the pixel number within the character row we need TAX \ to draw LDA TWOS,X \ Fetch a mode 4 1-pixel byte with the pixel position \ at X
 EOR (SC),Y             \ Draw the pixel on-screen using EOR logic, so we can
 STA (SC),Y             \ remove it later without ruining the background that's
                        \ already on-screen

Code variation 13 of 20Related to the screen mode

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Cassette, Flight, Docked

6502SP, Master

Electron

LDA CTWOS+1,X \ Fetch a mode 5 1-pixel byte with the pixel position \ at X+1, so we can draw the right pixel of the dash
LDA CTWOS+2,X \ Fetch a mode 2 1-pixel byte with the pixel position \ at (X+1)/2, so we can draw the right pixel of the dash
JSR P%+3 \ Run the following code twice, incrementing X each \ time, so we draw a two-pixel dash INX \ Increment X to get the next pixel along LDA TWOS,X \ Fetch a mode 4 1-pixel byte with the pixel position \ at X

Code variation 14 of 20A variation in the comments only

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Cassette, Flight, Docked

6502SP, Master

Electron

BPL CP1 \ The CTWOS table has an extra row at the end of it that \ repeats the first value, %10001000, so if we have not \ fetched that value, then the right pixel of the dash \ is in the same character block as the left pixel, so \ jump to CP1 to draw it
BPL CP1 \ The CTWOS table has 2 extra rows at the end of it that \ repeat the first values, %10101010, so if we have not \ fetched that value, then the right pixel of the dash \ is in the same character block as the left pixel, so \ jump to CP1 to draw it
BPL CP1 \ The CTWOS table is followed by the TWOS2 table, whose \ first entry is %11000000, so if we have not just \ fetched that value, then the right pixel of the dash \ is in the same character block as the left pixel, so \ jump to CP1 to draw it

Code variation 15 of 20Related to the screen mode

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Cassette, Flight, Docked, 6502SP, Master

Electron

LDA SC \ Otherwise the left pixel we drew was at the last ADC #8 \ position of four in this character block, so we add STA SC \ 8 to the screen address to move onto the next block \ along (as there are 8 bytes in a character block). \ The C flag was cleared above, so this ADC is correct
LDA SC \ Otherwise the left pixel we drew was at the last CLC \ position of four in this character block, so we add ADC #8 \ 8 to the screen address to move onto the next block STA SC \ along (as there are 8 bytes in a character block)

Code variation 16 of 20Related to the screen mode

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

6502SP, Master, Electron

BCC P%+4 \ If the addition we just did overflowed, then increment INC SC+1 \ the high byte of SC(1 0), as this means we just moved \ into the right half of the screen row

Code variation 17 of 20Related to the screen mode

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Cassette, Flight, Docked

6502SP, Master

Electron

LDA CTWOS+1,X \ Refetch the mode 5 1-pixel byte, as we just overwrote \ A (the byte will still be the fifth byte from the \ table, which is correct as we want to draw the \ leftmost pixel in the next character along as the \ dash's right pixel)
LDA CTWOS+2,X \ Refetch the mode 2 1-pixel byte, as we just overwrote \ A (the byte will still be the fifth or sixth byte from \ the table, which is correct as we want to draw the \ leftmost pixel in the next character along as the \ dash's right pixel)
LDA TWOS,X \ Refetch the mode 4 1-pixel byte from before, as we \ just overwrote A
.CP1

Code variation 18 of 20Related to the screen mode

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master

AND COL \ Apply the colour mask to the pixel byte, as above

Code variation 19 of 20Specific to an individual platform

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

Master

STA R \ Store the dash's right pixel byte in R

Code variation 20 of 20A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Master

Electron

EOR (SC),Y \ Draw the dash's right pixel according to the mask in STA (SC),Y \ A, with the colour in COL, using EOR logic, just as \ above
EOR (SC),Y \ Draw the dash's right pixel according to the mask in STA (SC),Y \ A, using EOR logic, just as above
 RTS                    \ Return from the subroutine

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