Version analysis of CLYNS - Elite on the 6502

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.

       Name: CLYNS
       Type: Subroutine
   Category: Drawing the screen

Code variation 1 of 14A variation in the comments only

This variation is blank in the 6502 Second Processor version.

Cassette, Flight, Docked, Master, Electron
    Summary: Clear the bottom three text rows of the space view

Code variation 2 of 14A variation in the comments only

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

Cassette, Flight, Docked, Electron
 This routine clears some space at the bottom of the screen and moves the text
 cursor to column 1, row 20.

Code variation 3 of 14A variation in the comments only

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

Cassette, Flight, Docked, Electron
 Specifically, it zeroes the following screen locations:

   &7507 to &75F0
   &7607 to &76F0
   &7707 to &77F0

 which clears the three bottom text rows of the mode 4 screen (rows 21 to 23),
 clearing each row from text column 1 to 30 (so it doesn't overwrite the box
 border in columns 0 and 32, or the last usable column in column 31).


 Returns:

   A                    A is set to 0

   Y                    Y is set to 0

Code variation 4 of 14A variation in the comments only

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

Electron
 Other entry points:

   SC5                  Contains an RTS


.CLYNS

Code variation 5 of 14Specific to an individual platform

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

Master
 STZ DLY                \ Set the delay in DLY to 0, to indicate that we are
                        \ no longer showing an in-flight message, so any new
                        \ in-flight messages will be shown instantly

 STZ de                 \ Clear de, the flag that appends " DESTROYED" to the
                        \ end of the next text token, so that it doesn't

Code variation 6 of 14Specific to an individual platform

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

Docked, Master
 LDA #%11111111         \ Set DTW2 = %11111111 to denote that we are not
 STA DTW2               \ currently printing a word

Code variation 7 of 14Specific to an individual platform

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

Master
 LDA #%10000000         \ Set bit 7 of QQ17 to switch standard tokens to
 STA QQ17               \ Sentence Case

Code variation 8 of 14Specific to an individual platform

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

Electron
 JSR BORDER             \ Redraw the space view's border, which removes it
                        \ from the screen

 LDX #&71               \ Call LYN with X = &71 to clear the screen from page
 JSR LYN                \ &71 to page &75, which clears the bottom three lines
                        \ of the screen

 JSR BORDER             \ Redraw the space view's border

 LDA #20                \ Move the text cursor to row 20, near the bottom of
 STA YC                 \ the screen

Code variation 9 of 14Related to the screen mode

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

Cassette, Flight
Docked
Master
6502SP
Electron
 LDA #&75               \ Set the two-byte value in SC to &7507
 STA SC+1
 LDA #7
 STA SC

 JSR TT67               \ Print a newline, which will move the text cursor down
                        \ a line (to row 21) and back to column 1
 JSR TT67               \ Print a newline, which will move the text cursor down
                        \ a line (to row 21) and back to column 1

 LDA #&75               \ Set the two-byte value in SC to &7507
 STA SC+1
 LDA #7
 STA SC
 JSR TT67K              \ Print a newline

 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

 LDA #&6A               \ Set SC+1 = &6A, for the high byte of SC(1 0)
 STA SC+1

 LDA #0                 \ Set SC = 0, so now SC(1 0) = &6A00
 STA SC
 LDA #&6A               \ Set SC+1 = &6A, for the high byte of SC(1 0)
 STA SC+1

 JSR TT67               \ Print a newline

 LDA #0                 \ Set SC = 0, so now SC(1 0) = &6A00
 STA SC
 JSR TT67               \ Print a newline, which will move the text cursor down
                        \ a line (to row 21) and back to column 1

Code variation 10 of 14Related to the screen mode

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

Cassette, Docked
Flight
6502SP, Master
Electron
 LDA #0                 \ Call LYN to clear the pixels from &7507 to &75F0
 JSR LYN

 INC SC+1               \ Increment SC+1 so SC points to &7607

 JSR LYN                \ Call LYN to clear the pixels from &7607 to &76F0

 INC SC+1               \ Increment SC+1 so SC points to &7707

 INY                    \ Move the text cursor to column 1 (as LYN sets Y to 0)
 STY XC

                        \ Fall through into LYN to clear the pixels from &7707
                        \ to &77F0
 LDA #0                 \ Call LYN to clear the pixels from &7507 to &75F0
 JSR LYN

 INC SC+1               \ Increment SC+1 so SC points to &7607

 INY                    \ Move the text cursor to column 1 (as LYN sets Y to 0)
 STY XC

                        \ Fall through into LYN to clear the pixels from &7707
                        \ to &77F0
 LDX #3                 \ We want to clear three text rows, so set a counter in
                        \ X for 3 rows

.CLYL

 LDY #8                 \ We want to clear each text row, starting from the
                        \ left, but we don't want to overwrite the border, so we
                        \ start from the second character block, which is byte
                        \ #8 from the edge, so set Y to 8 to act as the byte
                        \ counter within the row

.EE2

 STA (SC),Y             \ Zero the Y-th byte from SC(1 0), which clears it by
                        \ setting it to colour 0, black

 INY                    \ Increment the byte counter in Y

 BNE EE2                \ Loop back to EE2 to blank the next byte along, until
                        \ we have done one page's worth (from byte #8 to #255)

 INC SC+1               \ We have just finished the first page - which covers
                        \ the left half of the text row - so we increment SC+1
                        \ so SC(1 0) points to the start of the next page, or
                        \ the start of the right half of the row

 STA (SC),Y             \ Clear the byte at SC(1 0), as that won't be caught by
                        \ the next loop

 LDY #247               \ The second page covers the right half of the text row,
                        \ and as before we don't want to overwrite the border,
                        \ which we can do by starting from the last-but-one
                        \ character block and working our way left towards the
                        \ centre of the row. The last-but-one character block
                        \ ends at byte 247 (that's 255 - 8, as each character
                        \ is 8 bytes), so we put this in Y to act as a byte
                        \ counter, as before
 LDY #1                 \ Move the text cursor to column 1
 STY XC

 DEY                    \ Set Y = 0, so the subroutine returns with this value
 TYA

.SC5

 RTS                    \ Return from the subroutine

Code variation 11 of 14Specific to an individual platform

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

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

Master
6502SP
{
.EE3                    \ This label is a duplicate of a label in TT23 (which is
                        \ why we need to surround it with braces, as BeebAsm
                        \ doesn't allow us to redefine labels, unlike BBC
                        \ BASIC)

 STA (SC),Y             \ Zero the Y-th byte from SC(1 0), which clears it by
                        \ setting it to colour 0, black

 DEY                    \ Decrement the byte counter in Y

 BNE EE3                \ Loop back to EE2 to blank the next byte to the left,
                        \ until we have done one page's worth (from byte #247 to
                        \ #1)
}
.EE3

 STA (SC),Y             \ Zero the Y-th byte from SC(1 0), which clears it by
                        \ setting it to colour 0, black

 DEY                    \ Decrement the byte counter in Y

 BNE EE3                \ Loop back to EE2 to blank the next byte to the left,
                        \ until we have done one page's worth (from byte #247 to
                        \ #1)

Code variation 12 of 14Related to the screen mode

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

6502SP, Master
 INC SC+1               \ We have now blanked a whole text row, so increment
                        \ SC+1 so that SC(1 0) points to the next row

 DEX                    \ Decrement the row counter in X

 BNE CLYL               \ Loop back to blank another row, until we have done the
                        \ number of rows in X

Code variation 13 of 14A variation in the comments only

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

6502SP
\INX                    \ These instructions are commented out in the original
\STX SC                 \ source

Code variation 14 of 14Specific to an individual platform

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

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

Master
6502SP
 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

 LDA #0                 \ Set A = 0 as this is a return value for this routine

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