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elite-teletext-lines.asm
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elite-teletext-lines.asm
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\ ******************************************************************************
\
\ TELETEXT ELITE LINE ROUTINES
\
\ Elite was written by Ian Bell and David Braben and is copyright Acornsoft 1984
\
\ The code on this site has been reconstructed from a disassembly of the version
\ released on Ian Bell's personal website at http://www.elitehomepage.org/
\
\ The commentary is copyright Mark Moxon, and any misunderstandings or mistakes
\ in the documentation are entirely my fault
\
\ The terminology and notations used in this commentary are explained at
\ https://elite.bbcelite.com/terminology
\
\ The deep dive articles referred to in this commentary can be found at
\ https://elite.bbcelite.com/deep_dives
\
\ ------------------------------------------------------------------------------
\
\ The code in this section is taken from the Bitshifters' sixel line-drawing
\ routines here:
\
\ https://github.com/bitshifters/teletextr/blob/master/lib/bresenham.asm
\
\ It has been reformatted, but the core routines and comments are mostly
\ unchanged from the original.
\
\ ******************************************************************************
\ ******************************************************************************
\
\ Name: rtw
\ Type: Workspace
\ Category: Teletext Elite
\ Summary: Variables for use in the Bresenham routine below, based on
\ routines by Rich Talbot-Watkins (hence "rtw")
\
\ ******************************************************************************
.rtw_startx
SKIP 1
.rtw_starty
SKIP 1
.rtw_endx
SKIP 1
.rtw_endy
SKIP 1
.rtw_dx
SKIP 1
.rtw_dy
SKIP 1
.rtw_accum
SKIP 1
.rtw_count
SKIP 1
\ ******************************************************************************
\
\ Name: MoveToSixel
\ Type: Subroutine
\ Category: Teletext Elite
\ Summary: Set the position of the graphics cursor to (X, Y)
\
\ ------------------------------------------------------------------------------
\
\ Arguments:
\
\ X Sixel x-coordinate to move to
\
\ Y Sixel y-coordinate to move to
\
\ ******************************************************************************
.MoveToSixel
LDA rtw_startx \ Copy the current start coordinate to the end
STA rtw_endx \ coordinate
LDA rtw_starty
STA rtw_endy
STX rtw_startx \ Set the start coordinate to (X, Y)
STY rtw_starty
RTS \ Return from the subroutine
\ ******************************************************************************
\
\ Name: DrawToSixel
\ Type: Subroutine
\ Category: Teletext Elite
\ Summary: Plot a mode 7 sixel line from the graphics cursor to (X, Y)
\
\ ------------------------------------------------------------------------------
\
\ Arguments:
\
\ X Sixel x-coordinate to draw to
\
\ Y Sixel y-coordinate to draw to
\
\ ******************************************************************************
.DrawToSixel
LDA rtw_startx \ Copy the current start coordinate to the end
STA rtw_endx \ coordinate
LDA rtw_starty
STA rtw_endy
STX rtw_startx \ Set the start coordinate to (X, Y)
STY rtw_starty
SEC \ Calc dx = ABS(startx - endx)
LDA rtw_startx
TAX
SBC rtw_endx
BCS posdx
EOR #255
ADC #1
.posdx
STA rtw_dx
PHP \ C=0 if dir of startx -> endx is positive,
\ otherwise C=1
SEC \ Calc dy = ABS(starty - endy)
LDA rtw_starty
TAY
SBC rtw_endy
BCS posdy
EOR #255
ADC #1
.posdy
STA rtw_dy
PHP \ C=0 if dir of starty -> endy is positive,
\ otherwise C=1
ORA rtw_dx \ Coincident start and end points exit early
BNE nonzero
PLP \ Safe exit for coincident points
PLP
RTS
.nonzero
LDA rtw_dy \ Determine which type of line it is
CMP rtw_dx
BCC shallowline
.steepline
\ Self-modify code so that line progresses according to
\ direction remembered earlier
PLP \ C=sign of dy
LDA #&C8 \ INY (goingdown)
BCC P%+4
LDA #&88 \ DEY (goingup)
STA goingupdown
PLP \ C=sign of dx
LDA #&E8 \ INX (goingright)
BCC P%+4
LDA #&CA \ DEX (goingleft)
STA goingleftright
LDA rtw_dy \ Initialise accumulator for 'steep' line
STA rtw_count
LSR A
.steeplineloop
STA rtw_accum
JSR PlotSixelClipped \ Plot sixel
DEC rtw_count \ Check if done
BNE goingupdown
.exitline
RTS
.goingupdown
\ Move up to next line
NOP \ Self-modified to INY (goingdown) or DEY (goingup)
.movetonextcolumn
\ Check move to next pixel column
SEC
LDA rtw_accum
SBC rtw_dx
BCS steeplineloop
ADC rtw_dy
.goingleftright
\ Move left or right to next pixel column
NOP \ Self-modifed to INX (goingright) or DEX (goingleft)
JMP steeplineloop
.shallowline
\ Self-modify code so that line progresses according to
\ direction remembered earlier
PLP \ C=sign of dy
LDA #&C8 \ INY (goingdown)
BCC P%+4
LDA #&88 \ DEY (goingup)
STA goingupdown2
PLP \ C=sign of dx
LDA #&E8 \ INX (goingright)
BCC P%+4
LDA #&CA \ DEX (goingleft)
STA goingleftright2
LDA rtw_dx \ Initialise accumulator for 'steep' line
STA rtw_count
LSR A
.shallowlineloop
STA rtw_accum
JSR PlotSixelClipped \ Plot sixel in cached byte
DEC rtw_count \ Check if done
BNE goingleftright2
.exitline2
RTS
.goingleftright2
\ Move left or right to next pixel column
NOP \ Self-modifed to INX (goingright) or DEX (goingleft)
.movetonextline
\ Check whether we move to the next line
SEC
LDA rtw_accum
SBC rtw_dy
BCS shallowlineloop
ADC rtw_dx
.goingupdown2
\ Move down or up to next line
NOP \ Self-modified to INY (goingdown) or DEY (goingup)
JMP shallowlineloop