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⌂ ∞ ▽ Elite on the BBC Micro and NES

Moving: MVEIT (Part 3 of 9)

[BBC Micro cassette version]

       Name: MVEIT (Part 3 of 9)                                     [Show more]
       Type: Subroutine
   Category: Moving
    Summary: Move current ship: Move ship forward according to its speed

    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file




 This routine has multiple stages. This stage does the following:

   * Move the ship forward (along the vector pointing in the direction of
     travel) according to its speed:

     (x, y, z) += nosev_hi * speed / 64



 LDA INWK+27            \ Set Q = the ship's speed byte #27 * 4
 ASL A
 ASL A
 STA Q

 LDA INWK+10            \ Set A = |nosev_x_hi|
 AND #%01111111

 JSR FMLTU              \ Set R = A * Q / 256
 STA R                  \       = |nosev_x_hi| * speed / 64

 LDA INWK+10            \ If nosev_x_hi is positive, then:
 LDX #0                 \
 JSR MVT1-2             \   (x_sign x_hi x_lo) = (x_sign x_hi x_lo) + R
                        \
                        \ If nosev_x_hi is negative, then:
                        \
                        \   (x_sign x_hi x_lo) = (x_sign x_hi x_lo) - R
                        \
                        \ So in effect, this does:
                        \
                        \   (x_sign x_hi x_lo) += nosev_x_hi * speed / 64

 LDA INWK+12            \ Set A = |nosev_y_hi|
 AND #%01111111

 JSR FMLTU              \ Set R = A * Q / 256
 STA R                  \       = |nosev_y_hi| * speed / 64

 LDA INWK+12            \ If nosev_y_hi is positive, then:
 LDX #3                 \
 JSR MVT1-2             \   (y_sign y_hi y_lo) = (y_sign y_hi y_lo) + R
                        \
                        \ If nosev_y_hi is negative, then:
                        \
                        \   (y_sign y_hi y_lo) = (y_sign y_hi y_lo) - R
                        \
                        \ So in effect, this does:
                        \
                        \   (y_sign y_hi y_lo) += nosev_y_hi * speed / 64

 LDA INWK+14            \ Set A = |nosev_z_hi|
 AND #%01111111

 JSR FMLTU              \ Set R = A * Q / 256
 STA R                  \       = |nosev_z_hi| * speed / 64

 LDA INWK+14            \ If nosev_y_hi is positive, then:
 LDX #6                 \
 JSR MVT1-2             \   (z_sign z_hi z_lo) = (z_sign z_hi z_lo) + R
                        \
                        \ If nosev_z_hi is negative, then:
                        \
                        \   (z_sign z_hi z_lo) = (z_sign z_hi z_lo) - R
                        \
                        \ So in effect, this does:
                        \
                        \   (z_sign z_hi z_lo) += nosev_z_hi * speed / 64