Skip to navigation

Elite on the BBC Micro

Version analysis of LOIN (Part 1 of 7)

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

Code variations between these versions are shown below.

Name: LOIN (Part 1 of 7) Type: Subroutine Category: Drawing lines Summary: Draw a line: Calculate the line gradient in the form of deltas Deep dive: Bresenham's line algorithm
This routine draws a line from (X1, Y1) to (X2, Y2). It has multiple stages. This stage calculates the line deltas. Arguments: X1 The screen x-coordinate of the start of the line Y1 The screen y-coordinate of the start of the line X2 The screen x-coordinate of the end of the line Y2 The screen y-coordinate of the end of the line

Code variation 1 of 9A variation in the comments only

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

Cassette, Flight, Docked, Electron

Returns: Y Y is preserved Other entry points: LL30 LL30 is a synonym for LOIN and draws a line from (X1, Y1) to (X2, Y2)

Code variation 2 of 9Minor and very low-impact

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

Master

JMP HLOIN3 \ This instruction doesn't appear to be used anywhere

Code variation 3 of 9A variation in the labels only

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

Cassette, Flight, Docked, Electron

.LL30 SKIP 0 \ LL30 is a synomym for LOIN \
                        \ In the cassette and disc versions of Elite, LL30 and
                        \ LOIN are synonyms for the same routine, presumably
                        \ because the two developers each had their own line
                        \ routines to start with, and then chose one of them for
                        \ the final game

Code variation 4 of 9A variation in the comments only

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

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

Master

6502SP

\ \ In the BBC Master version, there are two different \ routines: LL30 draws a one-segment line, while LOIN \ draws multi-segment lines
\ \ In the 6502 Second Processor version, there are three \ different routines. In the parasite, LL30 draws a \ one-segment line, while LOIN draws multi-segment \ lines. Both of these ask the I/O processor to do the \ actual drawing, and it uses a routine called... wait \ for it... LOIN \ \ This, then, is the I/O processor's LOIN routine, which \ is not the same as LL30, or the other LOIN. Got that?
.LOIN

Code variation 5 of 9Specific to an individual platform

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

Cassette, Flight, Docked, Electron

STY YSAV \ Store Y into YSAV, so we can preserve it across the \ call to this subroutine
 LDA #128               \ Set S = 128, which is the starting point for the
 STA S                  \ slope error (representing half a pixel)

Code variation 6 of 9Related to the screen mode

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

Electron

STA SC \ Set SC = 128 for use in the pixel calculations below
 ASL A                  \ Set SWAP = 0, as %10000000 << 1 = 0
 STA SWAP

 LDA X2                 \ Set A = X2 - X1
 SBC X1                 \       = delta_x
                        \
                        \ This subtraction works as the ASL A above sets the C
                        \ flag

 BCS LI1                \ If X2 > X1 then A is already positive and we can skip
                        \ the next three instructions

 EOR #%11111111         \ Negate the result in A by flipping all the bits and
 ADC #1                 \ adding 1, i.e. using two's complement to make it
                        \ positive

Code variation 7 of 9Minor and very low-impact

This variation is blank in the Electron version.

Cassette, Flight, Docked, 6502SP, Master

SEC \ Set the C flag, ready for the subtraction below
.LI1

 STA P                  \ Store A in P, so P = |X2 - X1|, or |delta_x|

Code variation 8 of 9Minor and very low-impact

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

Electron

SEC \ Set the C flag, ready for the subtraction below
 LDA Y2                 \ Set A = Y2 - Y1
 SBC Y1                 \       = delta_y
                        \
                        \ This subtraction works as we either set the C flag
                        \ above, or we skipped that SEC instruction with a BCS

Code variation 9 of 9Specific to an individual platform

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

6502SP

BEQ HLOIN2 \ If A = 0 then Y1 = Y2, which means the line is \ horizontal, so jump to HLOIN2 to draw a horizontal \ line instead of applying Bresenham's line algorithm
 BCS LI2                \ If Y2 > Y1 then A is already positive and we can skip
                        \ the next two instructions

 EOR #%11111111         \ Negate the result in A by flipping all the bits and
 ADC #1                 \ adding 1, i.e. using two's complement to make it
                        \ positive

.LI2

 STA Q                  \ Store A in Q, so Q = |Y2 - Y1|, or |delta_y|

 CMP P                  \ If Q < P, jump to STPX to step along the x-axis, as
 BCC STPX               \ the line is closer to being horizontal than vertical

 JMP STPY               \ Otherwise Q >= P so jump to STPY to step along the
                        \ y-axis, as the line is closer to being vertical than
                        \ horizontal

RSS Feed