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Elite on the BBC Micro

Version analysis of PLUT / PU1

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

Code variations between these versions are shown below.

Code variation 1 of 4A variation in the comments only

This variation is blank in the Master version.

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

Cassette, 6502SP, Electron

Flight

Name: PLUT
Name: PU1
Type: Subroutine Category: Flight Summary: Flip the coordinate axes for the four different views Deep dive: Flipping axes between space views
This routine flips the relevant geometric axes in INWK depending on which view we are looking through (front, rear, left, right).

Code variation 2 of 4A variation in the comments only

This variation is blank in the Disc (flight) and Master versions.

Cassette, 6502SP, Electron

Other entry points: PU1-1 Contains an RTS

Code variation 3 of 4Specific to an individual platform

This variation is blank in the Disc (flight) version.

Cassette, 6502SP, Master, Electron

.PLUT LDX VIEW \ Load the current view into X: \ \ 0 = front \ 1 = rear \ 2 = left \ 3 = right

Code variation 4 of 4Minor and very low-impact

This variation is blank in the Disc (flight) version.

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

Cassette, Electron

6502SP, Master

BNE PU1 \ If the current view is the front view, return from the RTS \ subroutine, as the geometry in INWK is already correct
BEQ PU2-1 \ If the current view is the front view, return from the \ subroutine (PU2-1 contains an RTS), as the geometry in \ INWK is already correct
.PU1

 DEX                    \ Decrement the view, so now:
                        \
                        \   0 = rear
                        \   1 = left
                        \   2 = right

 BNE PU2                \ If the current view is left or right, jump to PU2,
                        \ otherwise this is the rear view, so continue on

 LDA INWK+2             \ Flip the sign of x_sign
 EOR #%10000000
 STA INWK+2

 LDA INWK+8             \ Flip the sign of z_sign
 EOR #%10000000
 STA INWK+8

 LDA INWK+10            \ Flip the sign of nosev_x_hi
 EOR #%10000000
 STA INWK+10

 LDA INWK+14            \ Flip the sign of nosev_z_hi
 EOR #%10000000
 STA INWK+14

 LDA INWK+16            \ Flip the sign of roofv_x_hi
 EOR #%10000000
 STA INWK+16

 LDA INWK+20            \ Flip the sign of roofv_z_hi
 EOR #%10000000
 STA INWK+20

 LDA INWK+22            \ Flip the sign of sidev_x_hi
 EOR #%10000000
 STA INWK+22

 LDA INWK+26            \ Flip the sign of roofv_z_hi
 EOR #%10000000
 STA INWK+26

 RTS                    \ Return from the subroutine

.PU2

                        \ We enter this with X set to the view, as follows:
                        \
                        \   1 = left
                        \   2 = right

 LDA #0                 \ Set RAT2 = 0 (left view) or -1 (right view)
 CPX #2
 ROR A
 STA RAT2

 EOR #%10000000         \ Set RAT = -1 (left view) or 0 (right view)
 STA RAT

 LDA INWK               \ Swap x_lo and z_lo
 LDX INWK+6
 STA INWK+6
 STX INWK

 LDA INWK+1             \ Swap x_hi and z_hi
 LDX INWK+7
 STA INWK+7
 STX INWK+1

 LDA INWK+2             \ Swap x_sign and z_sign
 EOR RAT                \ If left view, flip sign of new z_sign
 TAX                    \ If right view, flip sign of new x_sign
 LDA INWK+8
 EOR RAT2
 STA INWK+2
 STX INWK+8

 LDY #9                 \ Swap nosev_x_lo and nosev_z_lo
 JSR PUS1               \ Swap nosev_x_hi and nosev_z_hi
                        \ If left view, flip sign of new nosev_z_hi
                        \ If right view, flip sign of new nosev_x_hi

 LDY #15                \ Swap roofv_x_lo and roofv_z_lo
 JSR PUS1               \ Swap roofv_x_hi and roofv_z_hi
                        \ If left view, flip sign of new roofv_z_hi
                        \ If right view, flip sign of new roofv_x_hi

 LDY #21                \ Swap sidev_x_lo and sidev_z_lo
                        \ Swap sidev_x_hi and sidev_z_hi
                        \ If left view, flip sign of new sidev_z_hi
                        \ If right view, flip sign of new sidev_x_hi

.PUS1

 LDA INWK,Y             \ Swap the low x and z bytes for the vector in Y:
 LDX INWK+4,Y           \
 STA INWK+4,Y           \   * For Y =  9 swap nosev_x_lo and nosev_z_lo
 STX INWK,Y             \   * For Y = 15 swap roofv_x_lo and roofv_z_lo
                        \   * For Y = 21 swap sidev_x_lo and sidev_z_lo

 LDA INWK+1,Y           \ Swap the high x and z bytes for the offset in Y:
 EOR RAT                \
 TAX                    \   * If left view, flip sign of new z-coordinate
 LDA INWK+5,Y           \   * If right view, flip sign of new x-coordinate
 EOR RAT2
 STA INWK+1,Y
 STX INWK+5,Y

                        \ Fall through into LOOK1 to return from the subroutine

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