Version analysis of HLOIN - 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: HLOIN
       Type: Subroutine
   Category: Drawing lines

Code variation 1 of 38A variation in the comments only

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

Cassette, Flight, Docked, Master, Electron
6502SP
    Summary: Draw a horizontal line from (X1, Y1) to (X2, Y1)
    Summary: Implement the OSWORD 247 command (draw the sun lines in the
             horizontal line buffer in orange)

Code variation 2 of 38A variation in the comments only

This variation is blank in the 6502 Second Processor version.

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

Cassette, Flight, Docked, Electron
Master
  Deep dive: Drawing monochrome pixels on the BBC Micro
  Deep dive: Drawing colour pixels on the BBC Micro

Code variation 3 of 38A variation in the comments only

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

Cassette, Flight, Docked, Electron
Master
6502SP
 We do not draw a pixel at the right end of the line.


 Returns:

   Y                    Y is preserved
 This routine draws a horizontal orange line in the space view.

 We do not draw a pixel at the right end of the line.


 Returns:

   Y                    Y is preserved


 Other entry points:

   HLOIN3               Draw a line from (X, Y1) to (X2, Y1) in the colour given
                        in A
 This routine is run when the parasite sends an OSWORD 247 command with
 parameters in the block at OSSC(1 0). It draws a horizontal orange line (or a
 collection of lines) in the space view.

 The parameters match those put into the HBUF block in the parasite. Each line
 is drawn from (X1, Y1) to (X2, Y1), and lines are drawn in orange.

 We do not draw a pixel at the right end of the line.


 Arguments:

   OSSC(1 0)            A parameter block as follows:

                          * Byte #0 = The size of the parameter block being sent

                          * Byte #2 = The x-coordinate of the first line's
                                      starting point

                          * Byte #3 = The x-coordinate of the first line's end
                                      point

                          * Byte #4 = The y-coordinate of the first line

                          * Byte #5 = The x-coordinate of the second line's
                                      starting point

                          * Byte #6 = The x-coordinate of the second line's end
                                      point

                          * Byte #7 = The y-coordinate of the second line

                        and so on


 Other entry points:

   HLOIN3               Draw a line from (X1, Y1) to (X2, Y1) in the current
                        colour (we need to set Q = Y2 + 1 before calling
                        HLOIN3 so only one line is drawn)


.HLOIN

Code variation 4 of 38Related to the Electron version

The Electron doesn't have a dedicated routine for drawing horizontal lines, unlike the other versions; instead, it just uses the normal line-drawing routine, and sets the y-coordinates to be the same.

See below for more variations related to this code.

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

Cassette, Flight, Docked
Master
6502SP
Electron
 STY YSAV               \ Store Y into YSAV, so we can preserve it across the
                        \ call to this subroutine

 LDX X1                 \ Set X = X1
 LDA Y1                 \ Set A = Y1, the pixel y-coordinate of the line
 LDY #0                 \ Fetch byte #0 from the parameter block (which gives
 LDA (OSSC),Y           \ size of the parameter block) and store it in Q
 STA Q

 INY                    \ Increment Y to point to byte #2, which is where the
 INY                    \ line coordinates start

.HLLO

 LDA (OSSC),Y           \ Fetch the Y-th byte from the parameter block (the
 STA X1                 \ line's X1 coordinate) and store it in X1 and X
 TAX

 INY                    \ Fetch the Y+1-th byte from the parameter block (the
 LDA (OSSC),Y           \ line's X2 coordinate) and store it in X2
 STA X2

 INY                    \ Fetch the Y+2-th byte from the parameter block (the
 LDA (OSSC),Y           \ line's Y1 coordinate) and store it in Y1
 STA Y1

 STY Y2                 \ Store the parameter block offset for this line's Y1
                        \ coordinate in Y2, so we know where to fetch the next
                        \ line from in the parameter block once we have drawn
                        \ this one
 LDX Y1                 \ Set Y2 = Y1, so we can use the normal line-drawing
 STX Y2                 \ routine to draw a horizontal line

Code variation 5 of 38Related to the screen mode

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
 AND #3                 \ Set A to the correct order of red/yellow pixels to
 TAX                    \ make this line an orange colour (by using bits 0-1 of
 LDA orange,X           \ the pixel y-coordinate as the index into the orange
                        \ lookup table)

 STA COL                \ Store the correct orange colour in COL

.HLOIN3

 STY YSAV               \ Store Y into YSAV, so we can preserve it across the
                        \ call to this subroutine

 LDY #%00001111         \ Set bits 1 and 2 of the Access Control Register at
 STY VIA+&34            \ SHEILA &34 to switch screen memory into &3000-&7FFF

 LDX X1                 \ Set X = X1
 AND #3                 \ Set A to the correct order of red/yellow pixels to
 TAY                    \ make this line an orange colour (by using bits 0-1 of
 LDA orange,Y           \ the pixel y-coordinate as the index into the orange
                        \ lookup table)

.HLOIN3

 STA S                  \ Store the line colour in S

Code variation 6 of 38Specific to an individual platform

This variation is blank in the Electron version.

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

Cassette, Flight, Docked, Master
6502SP
 CPX X2                 \ If X1 = X2 then the start and end points are the same,
 BEQ HL6                \ so return from the subroutine (as HL6 contains an RTS)

 BCC HL5                \ If X1 < X2, jump to HL5 to skip the following code, as
                        \ (X1, Y1) is already the left point

 LDA X2                 \ Swap the values of X1 and X2, so we know that (X1, Y1)
 STA X1                 \ is on the left and (X2, Y1) is on the right
 STX X2

 TAX                    \ Set X = X1

.HL5

 DEC X2                 \ Decrement X2 so we do not draw a pixel at the end
                        \ point
 CPX X2                 \ If X1 = X2 then the start and end points are the same,
 BEQ HL6                \ so jump to HL6 to move on to the next line

 BCC HL5                \ If X1 < X2, jump to HL5 to skip the following code, as
                        \ (X1, Y1) is already the left point

 LDA X2                 \ Swap the values of X1 and X2, so we know that (X1, Y1)
 STA X1                 \ is on the left and (X2, Y1) is on the right
 STX X2

 TAX                    \ Set X = X1

.HL5

 DEC X2                 \ Decrement X2 so we do not draw a pixel at the end
                        \ point

Code variation 7 of 38Related to the screen mode

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
6502SP, Master
 LDA Y1                 \ Set A to the y-coordinate in Y1

 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 our horizontal line

 ORA #&60               \ As A < 32, this effectively adds &60 to A, which gives
                        \ us the screen address of the character row (as each
                        \ character row takes up 256 bytes, and the first
                        \ character row is at screen address &6000, or page &60)

 STA SCH                \ Store the page number of the character row in SCH, so
                        \ the high byte of SC is set correctly for drawing our
                        \ line

 LDA Y1                 \ Set A = Y1 mod 8, which is the pixel row within the
 AND #7                 \ character block at which we want to draw our line (as
                        \ each character block has 8 rows)
 LDY Y1                 \ Look up the page number of the character row that
 LDA ylookup,Y          \ contains the pixel with the y-coordinate in Y1, and
 STA SC+1               \ store it in SC+1, so the high byte of SC is set
                        \ correctly for drawing our line

 TYA                    \ Set A = Y1 mod 8, which is the pixel row within the
 AND #7                 \ character block at which we want to draw our line (as
                        \ each character block has 8 rows)

Code variation 8 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 STA SC                 \ Store this value in SC, so SC(1 0) now contains the
                        \ screen address of the far left end (x-coordinate = 0)
                        \ of the horizontal pixel row that we want to draw our
                        \ horizontal line on

Code variation 9 of 38Related to the screen mode

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
6502SP, Master
 TXA                    \ Set Y = bits 3-7 of X1
 AND #%11111000
 TAY
 TXA                    \ Set Y = 2 * bits 2-6 of X1
 AND #%11111100         \
 ASL A                  \ and shift bit 7 of X1 into the C flag
 TAY

 BCC P%+4               \ If bit 7 of X1 was set, so X1 > 127, increment the
 INC SC+1               \ high byte of SC(1 0) to point to the second page on
                        \ this screen row, as this page contains the right half
                        \ of the row

.HL1

Code variation 10 of 38Related to the screen mode

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
6502SP, Master
 TXA                    \ Set T = bits 3-7 of X1, which will contain the
 AND #%11111000         \ character number of the start of the line * 8
 STA T

 LDA X2                 \ Set A = bits 3-7 of X2, which will contain the
 AND #%11111000         \ character number of the end of the line * 8

 SEC                    \ Set A = A - T, which will contain the number of
 SBC T                  \ character blocks we need to fill - 1 * 8
 TXA                    \ Set T = bits 2-7 of X1, which will contain the
 AND #%11111100         \ character number of the start of the line * 4
 STA T

 LDA X2                 \ Set A = bits 2-7 of X2, which will contain the
 AND #%11111100         \ character number of the end of the line * 4

 SEC                    \ Set A = A - T, which will contain the number of
 SBC T                  \ character blocks we need to fill - 1 * 4

Code variation 11 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 BEQ HL2                \ If A = 0 then the start and end character blocks are
                        \ the same, so the whole line fits within one block, so
                        \ jump down to HL2 to draw the line

                        \ Otherwise the line spans multiple characters, so we
                        \ start with the left character, then do any characters
                        \ in the middle, and finish with the right character

Code variation 12 of 38Related to the screen mode

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
6502SP, Master
 LSR A                  \ Set R = A / 8, so R now contains the number of
 LSR A                  \ character blocks we need to fill - 1
 LSR A
 STA R

 LDA X1                 \ Set X = X1 mod 8, which is the horizontal pixel number
 AND #7                 \ within the character block where the line starts (as
 TAX                    \ each pixel line in the character block is 8 pixels
                        \ wide)
 LSR A                  \ Set R = A / 4, so R now contains the number of
 LSR A                  \ character blocks we need to fill - 1
 STA R

 LDA X1                 \ Set X = X1 mod 4, which is the horizontal pixel number
 AND #3                 \ within the character block where the line starts (as
 TAX                    \ each pixel line in the character block is 4 pixels
                        \ wide)

Code variation 13 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 LDA TWFR,X             \ Fetch a ready-made byte with X pixels filled in at the
                        \ right end of the byte (so the filled pixels start at
                        \ point X and go all the way to the end of the byte),
                        \ which is the shape we want for the left end of the
                        \ line

Code variation 14 of 38Related to the screen mode

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
 AND COL                \ Apply the pixel mask in A to the four-pixel block of
                        \ coloured pixels in COL, so we now know which bits to
                        \ set in screen memory to paint the relevant pixels in
                        \ the required colour
 AND S                  \ Apply the pixel mask in A to the four-pixel block of
                        \ coloured pixels in S, so we now know which bits to set
                        \ in screen memory to paint the relevant pixels in the
                        \ required colour

Code variation 15 of 38Related to the Electron version

See variation 4 above for details.

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

Cassette, Flight, Docked, 6502SP, Master
Electron
 EOR (SC),Y             \ Store this into screen memory at SC(1 0), using EOR
 STA (SC),Y             \ logic so it merges with whatever is already on-screen,
                        \ so we have now drawn the line's left cap

 TYA                    \ Set Y = Y + 8 so (SC),Y points to the next character
 ADC #8                 \ block along, on the same pixel row as before
 TAY
 JMP LL30               \ Draw a line from (X1, Y1) to (X2, Y2), which will be
                        \ horizontal because we set Y2 to Y1 above

Code variation 16 of 38Related to the screen mode

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

6502SP, Master
 BCS HL7                \ If the above addition overflowed, then we have just
                        \ crossed over from the left half of the screen into the
                        \ right half, so call HL7 to increment the high byte in
                        \ SC+1 so that SC(1 0) points to the page in screen
                        \ memory for the right half of the screen row. HL7 also
                        \ clears the C flag and jumps back to HL8, so this acts
                        \ like a conditional JSR instruction

.HL8

Code variation 17 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 LDX R                  \ Fetch the number of character blocks we need to fill
                        \ from R

 DEX                    \ Decrement the number of character blocks in X

 BEQ HL3                \ If X = 0 then we only have the last block to do (i.e.
                        \ the right cap), so jump down to HL3 to draw it

 CLC                    \ Otherwise clear the C flag so we can do some additions
                        \ while we draw the character blocks with full-width
                        \ lines in them

.HLL1

Code variation 18 of 38Related to the screen mode

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
Master
6502SP
 LDA #%11111111         \ Store a full-width eight-pixel horizontal line in
 EOR (SC),Y             \ SC(1 0) so that it draws the line on-screen, using EOR
 STA (SC),Y             \ logic so it merges with whatever is already on-screen
 LDA COL                \ Store a full-width four-pixel horizontal line of
 EOR (SC),Y             \ colour COL in SC(1 0) so that it draws the line
 STA (SC),Y             \ on-screen, using EOR logic so it merges with whatever
                        \ is already on-screen
 LDA S                  \ Store a full-width four-pixel horizontal line of
 EOR (SC),Y             \ colour S in SC(1 0) so that it draws the line
 STA (SC),Y             \ on-screen, using EOR logic so it merges with whatever
                        \ is already on-screen

Code variation 19 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 TYA                    \ Set Y = Y + 8 so (SC),Y points to the next character
 ADC #8                 \ block along, on the same pixel row as before
 TAY

Code variation 20 of 38Related to the screen mode

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

6502SP, Master
 BCS HL9                \ If the above addition overflowed, then we have just
                        \ crossed over from the left half of the screen into the
                        \ right half, so call HL9 to increment the high byte in
                        \ SC+1 so that SC(1 0) points to the page in screen
                        \ memory for the right half of the screen row. HL9 also
                        \ clears the C flag and jumps back to HL10, so this acts
                        \ like a conditional JSR instruction

.HL10

Code variation 21 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 DEX                    \ Decrement the number of character blocks in X

 BNE HLL1               \ Loop back to draw more full-width lines, if we have
                        \ any more to draw

.HL3

Code variation 22 of 38Related to the screen mode

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
6502SP, Master
 LDA X2                 \ Now to draw the last character block at the right end
 AND #7                 \ of the line, so set X = X2 mod 8, which is the
 TAX                    \ horizontal pixel number where the line ends
 LDA X2                 \ Now to draw the last character block at the right end
 AND #3                 \ of the line, so set X = X2 mod 3, which is the
 TAX                    \ horizontal pixel number where the line ends

Code variation 23 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 LDA TWFL,X             \ Fetch a ready-made byte with X pixels filled in at the
                        \ left end of the byte (so the filled pixels start at
                        \ the left edge and go up to point X), which is the
                        \ shape we want for the right end of the line

Code variation 24 of 38Related to the screen mode

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
 AND COL                \ Apply the pixel mask in A to the four-pixel block of
                        \ coloured pixels in COL, so we now know which bits to
                        \ set in screen memory to paint the relevant pixels in
                        \ the required colour
 AND S                  \ Apply the pixel mask in A to the four-pixel block of
                        \ coloured pixels in S, so we now know which bits to set
                        \ in screen memory to paint the relevant pixels in the
                        \ required colour

Code variation 25 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 EOR (SC),Y             \ Store this into screen memory at SC(1 0), using EOR
 STA (SC),Y             \ logic so it merges with whatever is already on-screen,
                        \ so we have now drawn the line's right cap

Code variation 26 of 38Related to Elite's use of the Tube

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
Master
6502SP
 LDY YSAV               \ Restore Y from YSAV, so that it's preserved across the
                        \ call to this subroutine
.HL6

 LDY #%00001001         \ Clear bits 1 and 2 of the Access Control Register at
 STY VIA+&34            \ SHEILA &34 to switch main memory back into &3000-&7FFF

 LDY YSAV               \ Restore Y from YSAV, so that it's preserved
.HL6

 LDY Y2                 \ Set Y to the parameter block offset for this line's Y1
                        \ coordinate, which we stored in Y2 before we drew the
                        \ line

 INY                    \ Increment Y so that it points to the first parameter
                        \ for the next line in the parameter block

 CPY Q                  \ If Y = Q then we have drawn all the lines in the
 BEQ P%+5               \ parameter block, so skip the next instruction to
                        \ return from the subroutine

 JMP HLLO               \ There is another line in the parameter block after the
                        \ one we just drew, so jump to HLLO with Y pointing to
                        \ the new line's coordinates, so we can draw it

Code variation 27 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 RTS                    \ Return from the subroutine

.HL2

                        \ If we get here then the entire horizontal line fits
                        \ into one character block

Code variation 28 of 38Related to the screen mode

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
6502SP, Master
 LDA X1                 \ Set X = X1 mod 8, which is the horizontal pixel number
 AND #7                 \ within the character block where the line starts (as
 TAX                    \ each pixel line in the character block is 8 pixels
                        \ wide)
 LDA X1                 \ Set X = X1 mod 4, which is the horizontal pixel number
 AND #3                 \ within the character block where the line starts (as
 TAX                    \ each pixel line in the character block is 4 pixels
                        \ wide)

Code variation 29 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 LDA TWFR,X             \ Fetch a ready-made byte with X pixels filled in at the
 STA T                  \ right end of the byte (so the filled pixels start at
                        \ point X and go all the way to the end of the byte)

Code variation 30 of 38Related to the screen mode

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
6502SP, Master
 LDA X2                 \ Set X = X2 mod 8, which is the horizontal pixel number
 AND #7                 \ where the line ends
 TAX
 LDA X2                 \ Set X = X2 mod 4, which is the horizontal pixel number
 AND #3                 \ where the line ends
 TAX

Code variation 31 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 LDA TWFL,X             \ Fetch a ready-made byte with X pixels filled in at the
                        \ left end of the byte (so the filled pixels start at
                        \ the left edge and go up to point X)

 AND T                  \ We now have two bytes, one (T) containing pixels from
                        \ the starting point X1 onwards, and the other (A)
                        \ containing pixels up to the end point at X2, so we can
                        \ get the actual line we want to draw by AND'ing them
                        \ together. For example, if we want to draw a line from

Code variation 32 of 38A variation in the comments only

This variation is blank in the Master and Electron versions.

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

Cassette, Flight, Docked
6502SP
                        \ point 2 to point 5 (within the row of 8 pixels
                        \ numbered from 0 to 7), we would have this:
                        \ point 1 to point 2 (within the row of 4 pixels
                        \ numbered from 0 to 3), we would have this:

Code variation 33 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
                        \
                        \   T       = %00111111
                        \   A       = %11111100
                        \   T AND A = %00111100
                        \
                        \ So we can stick T AND A in screen memory to get the
                        \ line we want, which is what we do here by setting
                        \ A = A AND T

Code variation 34 of 38Related to the screen mode

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
 AND COL                \ Apply the pixel mask in A to the four-pixel block of
                        \ coloured pixels in COL, so we now know which bits to
                        \ set in screen memory to paint the relevant pixels in
                        \ the required colour
 AND S                  \ Apply the pixel mask in A to the four-pixel block of
                        \ coloured pixels in S, so we now know which bits to set
                        \ in screen memory to paint the relevant pixels in the
                        \ required colour

Code variation 35 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 EOR (SC),Y             \ Store our horizontal line byte into screen memory at
 STA (SC),Y             \ SC(1 0), using EOR logic so it merges with whatever is
                        \ already on-screen

Code variation 36 of 38Related to Elite's use of the Tube

This variation is blank in the Electron version.

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

Cassette, Flight, Docked
Master
6502SP
 LDY YSAV               \ Restore Y from YSAV, so that it's preserved
 LDY #%00001001         \ Clear bits 1 and 2 of the Access Control Register at
 STY VIA+&34            \ SHEILA &34 to switch main memory back into &3000-&7FFF

 LDY YSAV               \ Restore Y from YSAV, so that it's preserved
 LDY Y2                 \ Set Y to the parameter block offset for this line's Y1
                        \ coordinate, which we stored in Y2 before we drew the
                        \ line

 INY                    \ Increment Y so that it points to the first parameter
                        \ for the next line in the parameter block

 CPY Q                  \ If Y = Q then we have drawn all the lines in the
 BEQ P%+5               \ parameter block, so skip the next instruction to
                        \ return from the subroutine

 JMP HLLO               \ There is another line in the parameter block after the
                        \ one we just drew, so jump to HLLO with Y pointing to
                        \ the new line's coordinates, so we can draw it

Code variation 37 of 38Specific to an individual platform

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master
 RTS                    \ Return from the subroutine

Code variation 38 of 38Related to the screen mode

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

6502SP, Master
.HL7

 INC SC+1               \ We have just crossed over from the left half of the
                        \ screen into the right half, so increment the high byte
                        \ in SC+1 so that SC(1 0) points to the page in screen
                        \ memory for the right half of the screen row

 CLC                    \ Clear the C flag (as HL7 is called with the C flag
                        \ set, which this instruction reverts)

 JMP HL8                \ Jump back to HL8, just after the instruction that
                        \ called HL7

.HL9

 INC SC+1               \ We have just crossed over from the left half of the
                        \ screen into the right half, so increment the high byte
                        \ in SC+1 so that SC(1 0) points to the page in screen
                        \ memory for the right half of the screen row

 CLC                    \ Clear the C flag (as HL9 is called with the C flag
                        \ set, which this instruction reverts)

 JMP HL10               \ Jump back to HL10, just after the instruction that
                        \ called HL9