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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).

Cassette, 6502SP, Electron

Other entry points: PU1-1 Contains an RTS

Cassette, 6502SP, Master, Electron

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.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