Drawing planets: PLANET

       Name: PLANET                                                  [Show more]
       Type: Subroutine
   Category: Drawing planets
    Summary: Draw the planet or sun
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * LL9 (Part 1 of 12) calls PLANET


 Arguments:

   INWK                 The planet or sun's ship data block


.PLANET

 LDA INWK+8             ; Set A = z_sign (the highest byte in the planet/sun's
                        ; coordinates)

;BMI PL2                ; This instruction is commented out in the original
                        ; source. It would remove the planet from the screen
                        ; when it's behind us

 CMP #48                ; If A >= 48 then the planet/sun is too far away to be
 BCS PL2                ; seen, so jump to PL2 to remove it from the screen,
                        ; returning from the subroutine using a tail call

 ORA INWK+7             ; Set A to 0 if both z_sign and z_hi are 0

 BEQ PL2                ; If both z_sign and z_hi are 0, then the planet/sun is
                        ; too close to be shown, so jump to PL2 to remove it
                        ; from the screen, returning from the subroutine using a
                        ; tail call

 JSR PROJ               ; Project the planet/sun onto the screen, returning the
                        ; centre's coordinates in K3(1 0) and K4(1 0)

 BCS PL2                ; If the C flag is set by PROJ then the planet/sun is
                        ; not visible on-screen, so jump to PL2 to remove it
                        ; from the screen, returning from the subroutine using
                        ; a tail call

 LDA #96                ; Set (A P+1 P) = (0 96 0) = 24576
 STA P+1                ;
 LDA #0                 ; This represents the planet/sun's radius at a distance
 STA P                  ; of z = 1

 JSR DVID3B2            ; Call DVID3B2 to calculate:
                        ;
                        ;   K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo)
                        ;              = (0 96 0) / z
                        ;              = 24576 / z
                        ;
                        ; so K now contains the planet/sun's radius, reduced by
                        ; the actual distance to the planet/sun. We know that
                        ; K+3 and K+2 will be 0, as the number we are dividing,
                        ; (0 96 0), fits into the two bottom bytes, so the
                        ; result is actually in K(1 0)

 LDA K+1                ; If the high byte of the reduced radius is zero, jump
 BEQ PL82               ; to PL82, as K contains the radius on its own

 LDA #248               ; Otherwise set K = 248, to round up the radius in
 STA K                  ; K(1 0) to the nearest integer (if we consider the low
                        ; byte to be the fractional part)

.PL82

 LDA TYPE               ; If the planet/sun's type has bit 0 clear, then it's
 LSR A                  ; either 128 or 130, which is a planet (the sun has type
 BCC PL9                ; 129, which has bit 0 set). So jump to PL9 to draw the
                        ; planet with radius K, returning from the subroutine
                        ; using a tail call

 JMP SUN                ; Otherwise jump to SUN to draw the sun with radius K,
                        ; returning from the subroutine using a tail call