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BBC Micro Elite

Drawing ships: LL9 (Part 1 of 11)

Name: LL9 (Part 1 of 11) [View in context] Type: Subroutine Category: Drawing ships Summary: Draw ship: Check if ship is exploding, check if ship is in front Deep dive: Drawing ships
This routine draws the current ship on the screen. This part checks to see if the ship is exploding, or if it should start exploding, and if it does it sets things up accordingly. It also does some basic checks to see if we can see the ship, and if not it removes it from the screen. In this code, XX1 is used to point to the current ship's data block at INWK (the two labels are interchangeable). Arguments: XX1 XX1 shares its location with INWK, which contains the zero-page copy of the data block for this ship from the K% workspace INF The address of the data block for this ship in workspace K% XX19(1 0) XX19(1 0) shares its location with INWK(34 33), which contains the ship line heap address pointer XX0 The address of the blueprint for this ship Other entry points: EE51 Remove the current ship from the screen, called from SHPPT before drawing the ship as a point LL81+2 Draw the contents of the ship lone heap, used to draw the ship as a dot from SHPPT LL10-1 Contains an RTS
.LL25 JMP PLANET \ Jump to the PLANET routine, returning from the \ subroutine using a tail call .LL9 LDA TYPE \ If the ship type is negative then this indicates a BMI LL25 \ planet or sun, so jump to PLANET via LL25 above LDA #31 \ Set XX4 = 31 to store the ship's distance for later STA XX4 \ comparison with the visibility distance. We will \ update this value below with the actual ship's \ distance if it turns out to be visible on-screen LDA #%00100000 \ If bit 5 of the ship's byte #31 is set, then the ship BIT XX1+31 \ is currently exploding, so jump down to EE28 BNE EE28 BPL EE28 \ If bit 7 of the ship's byte #31 is clear then the ship \ has not just been killed, so jump down to EE28 \ Otherwise bit 5 is clear and bit 7 is set, so the ship \ is not yet exploding but it has been killed, so we \ need to start an explosion ORA XX1+31 \ Clear bits 6 and 7 of the ship's byte #31, to stop the AND #%00111111 \ ship from firing its laser and to mark it as no longer STA XX1+31 \ having just been killed LDA #0 \ Set the ship's acceleration in byte #31 to 0, updating LDY #28 \ the byte in the workspace K% data block so we don't STA (INF),Y \ have to copy it back from INWK later LDY #30 \ Set the ship's pitch counter in byte #30 to 0, to stop STA (INF),Y \ the ship from pitching JSR EE51 \ Call EE51 to remove the ship from the screen \ We now need to set up a new explosion cloud. We \ initialise it with a size of 18 (which gets increased \ by 4 every time the cloud gets redrawn), and the \ explosion count (i.e. the number of particles in the \ explosion), which go into bytes 1 and 2 of the ship \ line heap. See DOEXP for more details of explosion \ clouds LDY #1 \ Set byte #1 of the ship line heap to 18, the initial LDA #18 \ size of the explosion cloud STA (XX19),Y LDY #7 \ Fetch byte #7 from the ship's blueprint, which LDA (XX0),Y \ determines the explosion count (i.e. the number of LDY #2 \ vertices used as origins for explosion clouds), and STA (XX19),Y \ store it in byte #2 of the ship line heap \LDA XX1+32 \ These instructions are commented out in the original \AND #&7F \ source \ The following loop sets bytes 3-6 of the of the ship \ line heap to random numbers .EE55 INY \ Increment Y (so the loop starts at 3) JSR DORND \ Set A and X to random numbers STA (XX19),Y \ Store A in the Y-th byte of the ship line heap CPY #6 \ Loop back until we have randomised the 6th byte BNE EE55 .EE28 LDA XX1+8 \ Set A = z_sign .EE49 BPL LL10 \ If A is positive, i.e. the ship is in front of us, \ jump down to LL10 .LL14 \ The following removes the ship from the screen by \ redrawing it (or, if it is exploding, by redrawing the \ explosion cloud). We call it when the ship is no \ longer on-screen, is too far away to be fully drawn, \ and so on LDA XX1+31 \ If bit 5 of the ship's byte #31 is clear, then the AND #%00100000 \ ship is not currently exploding, so jump down to EE51 BEQ EE51 \ to redraw its wireframe LDA XX1+31 \ The ship is exploding, so clear bit 3 of the ship's AND #%11110111 \ byte #31 to denote that the ship is no longer being STA XX1+31 \ drawn on-screen JMP DOEXP \ Jump to DOEXP to display the explosion cloud, which \ will remove it from the screen, returning from the \ subroutine using a tail call .EE51 LDA #%00001000 \ If bit 3 of the ship's byte #31 is clear, then there BIT XX1+31 \ is already nothing being shown for this ship, so BEQ LL10-1 \ return from the subroutine (as LL10-1 contains an RTS) EOR XX1+31 \ Otherwise flip bit 3 of byte #31 and store it (which STA XX1+31 \ clears bit 3 as we know it was set before the EOR), so \ this sets this ship as no longer being drawn on-screen JMP LL155 \ Jump to LL155 to draw the ship, which removes it from \ the screen, returning from the subroutine using a \ tail call \LL24 \ This label is commented out in the original source, \ and was presumably used to label the RTS which is \ actually called by LL10-1 above, not LL24 RTS \ Return from the subroutine