.PTCLS2 LDA #%101 ; Call SETL1 to set the 6510 input/output port to the JSR SETL1 ; following: ; ; * LORAM = 1 ; * HIRAM = 0 ; * CHAREN = 1 ; ; This sets the entire 64K memory map to RAM except for ; the I/O memory map at $D000-$DFFF, which gets mapped ; to registers in the VIC-II video controller chip, the ; SID sound chip, the two CIA I/O chips, and so on ; ; See the memory map at the top of page 264 in the ; Programmer's Reference Guide ; We now set up sprite 1, so we can use it to show the ; explosion burst as a colourful sprite (along with the ; usual cloud of explosion particles) LDA INWK+7 ; If z_hi >= 7 then set the following: CMP #7 ; LDA #%11111101 ; * A = %11111101 LDX #44 ; * X = 44 LDY #40 ; * Y = 40 BCS noexpand ; LDA #%11111111 ; otherwise set the following: LDX #32 ; LDY #30 ; * A = %11111111 ; * X = 32 ; * Y = 30 .noexpand STA VIC+$17 ; Store A in VIC registers $17 and $1D, which determine STA VIC+$1D ; whether sprites are double height and double width ; ; So this set sprite 1 to be standard width and height ; when z_hi >= 7 (as bit 1 of A is clear), or double ; width and height when z_hi < 7 (as bit 1 of A is set) ; ; We are about to use sprite 1 to display the explosion ; sprite, so this means that explosions that are far ; away will show a smaller sprite than explosions that ; are closer STX sprx ; Store X and Y in sprx and spry, to be used as the STY spry ; x- and y-coordinate offsets for the explosion sprite ; (i.e. the relative position of the sprite compared to ; the centre of the explosion, which needs to be set ; according to the size of the sprite) ; This part of the routine actually draws the explosion ; cloud LDY #0 ; Fetch byte #0 of the ship line heap, which contains LDA (XX19),Y ; the cloud size we stored above, and store it in Q STA Q INY ; Increment the index in Y to point to byte #1 LDA (XX19),Y ; Fetch byte #1 of the ship line heap, which contains ; the cloud counter. We are now going to process this ; into the number of particles in each vertex's cloud BPL P%+4 ; If the cloud counter < 128, then we are in the first ; half of the cloud's existence, so skip the next ; instruction EOR #$FF ; Flip the value of A so that in the second half of the ; cloud's existence, A counts down instead of up LSR A ; Divide A by 16 so that is has a maximum value of 7 LSR A LSR A LSR A ORA #1 ; Make sure A is at least 1 and store it in U, to STA U ; give us the number of particles in the explosion for ; each vertex INY ; Increment the index in Y to point to byte #2 LDA (XX19),Y ; Fetch byte #2 of the ship line heap, which contains STA TGT ; the explosion count for this ship (i.e. the number of ; vertices used as origins for explosion clouds) and ; store it in TGT LDA RAND+1 ; Fetch the current random number seed in RAND+1 and PHA ; store it on the stack, so we can re-randomise the ; seeds when we are done LDY #6 ; Set Y = 6 to point to the byte before the first vertex ; coordinate we stored on the ship line heap above (we ; increment it below so it points to the first vertex) .EXL52 LDX #3 ; We are about to fetch a pair of coordinates from the ; ship line heap, so set a counter in X for 4 bytes .EXL32 INY ; Increment the index in Y so it points to the next byte ; from the coordinate we are copying LDA (XX19),Y ; Copy the Y-th byte from the ship line heap to the X-th STA K3,X ; byte of K3 DEX ; Decrement the X index BPL EXL32 ; Loop back to EXL32 until we have copied all four bytes ; The above loop copies the vertex coordinates from the ; ship line heap to K3, reversing them as we go, so it ; sets the following: ; ; K3+3 = x_lo ; K3+2 = x_hi ; K3+1 = y_lo ; K3+0 = y_hi STY CNT ; Set CNT to the index that points to the next vertex on ; the ship line heap ; We now draw sprite 1, so it shows the explosion burst ; as a colourful sprite (along with the usual cloud of ; explosion particles) LDA K3+3 ; Set (A SC) = K3(2 3) + (0 sprx) CLC ; = (x_hi x_lo) + x-coordinate offset ADC sprx STA SC LDA K3+2 ADC #0 BMI yonk ; If the high byte of the result in (A SC) is negative, ; then the explosion is off the left edge of the screen, ; so jump to yonk to skip displaying the sprite CMP #2 ; If A >= 2 then (A SC) >= $200 (i.e. 512), so the BCS yonk ; explosion is way past the right edge of the screen, so ; jump to yonk to skip displaying the sprite TAX ; Set (X SC) = (A SC) ; ; so (X SC) contains the 9-bit x-coordinate of the ; explosion LDA K3+1 ; Set (A Y) = K3(0 1) + (0 spry) CLC ; = (y_hi y_lo) + y-coordinate offset ADC spry TAY LDA K3 ADC #0 BNE yonk ; If the high byte of the result in (A Y) is non-zero, ; then the explosion is either off the top of bottom ; edge of the screen, so jump to yonk to skip displaying ; the sprite CPY #2*Y+50 ; If the low byte of the result in (A Y) is greater than BCS yonk ; the height of the space view (2 * #Y) plus 50, then ; explosion is behind the dashboard (as it is more than ; 50 pixels below the top edge of the dashboard), so ; jump to yonk to skip displaying the sprite ; If we get here then the explosion is visible in the ; space view, so we now draw sprite 1, which contains ; the explosion sprite LDA VIC+$10 ; The 9-bit x-coordinate of the sprite is in (X SC), so AND #%11111101 ; set bit 1 of VIC register $10 when X is 1, or clear ORA exlook,X ; bit 1 of VIC register $10 when X is 1 (as VIC register STA VIC+$10 ; $10 contains the top bit of the x-coordinate for all ; eight sprites, with sprite 1's bit appearing in bit 1) ; ; The ORA exlook,X instruction is a neat way of setting ; bit 1 of A to the value of X, as exlook contains %00 ; and %10 (you could achieve the same effect by shifting ; X to the left by one place and OR'ing that, but that ; would require quite a bit of register shuffling on the ; 6502) LDX SC ; Set VIC registers $02 and $03 to the x-coordinate and STY VIC+$3 ; y-coordinate of sprite 1, which are in X and Y STX VIC+$2 LDA VIC+$15 ; Set bit 1 of VIC register $15 to enable sprite 1 so ORA #%00000010 ; the explosion sprite appears on-screen in the correct STA VIC+$15 ; position .yonk LDY #2 ; Set Y = 2, which we will use to point to bytes #3 to ; #6, after incrementing it ; This next loop copies bytes #3 to #6 from the ship ; line heap into the four random number seeds in RAND to ; RAND+3, EOR'ing them with the vertex index so they are ; different for every vertex. This enables us to ; generate random numbers for drawing each vertex that ; are random but repeatable, which we need when we ; redraw the cloud to remove it ; ; Note that we haven't actually set the values of bytes ; #3 to #6 in the ship line heap, so we have no idea ; what they are, we just use what's already there. But ; the fact that those bytes are stored for this ship ; means we can repeat the random generation of the ; cloud, which is the important bit .EXL22 INY ; Increment the index in Y so it points to the next ; random number seed to copy LDA (XX19),Y ; Fetch the Y-th byte from the ship line heap EOR CNT ; EOR with the vertex index, so the seeds are different ; for each vertex STA $FFFF,Y ; Y is going from 3 to 6, so this stores the four bytes ; in memory locations $02, $03, $04 and $05, which are ; the memory locations of RAND through RAND+3 CPY #6 ; Loop back to EXL22 until Y = 6, which means we have BNE EXL22 ; copied four bytes LDY U ; Set Y to the number of particles in the explosion for ; each vertex, which we stored in U above. We will now ; use this as a loop counter to iterate through all the ; particles in the explosion .EXL42 JSR DORND2 ; Set ZZ to a random number, making sure the C flag STA ZZ ; doesn't affect the outcome LDA K3+1 ; Set (A R) = (y_hi y_lo) STA R ; = y LDA K3 JSR EXS1 ; Set (A X) = (A R) +/- random * cloud size ; = y +/- random * cloud size BNE EX112 ; If A is non-zero, the particle is off-screen as the ; coordinate is bigger than 255), so jump to EX112 to do ; the next particle CPX #2*Y-1 ; If X > the y-coordinate of the bottom of the screen, BCS EX112 ; the particle is off the bottom of the screen, so jump ; to EX112 to do the next particle ; Otherwise X contains a random y-coordinate within the ; cloud STX Y1 ; Set Y1 = our random y-coordinate within the cloud LDA K3+3 ; Set (A R) = (x_hi x_lo) STA R LDA K3+2 JSR EXS1 ; Set (A X) = (A R) +/- random * cloud size ; = x +/- random * cloud size BNE EX42 ; If A is non-zero, the particle is off-screen as the ; coordinate is bigger than 255), so jump to EX42 to do ; the next particle LDA Y1 ; Set A = our random y-coordinate within the cloud JSR PIXEL ; Draw a point at screen coordinate (X, A) with the ; point size determined by the distance in ZZ .EX42 DEY ; Decrement the loop counter for the next particle BPL EXL42 ; Loop back to EXL42 until we have done all the ; particles in the cloud LDY CNT ; Set Y to the index that points to the next vertex on ; the ship line heap CPY TGT ; If Y < TGT, which we set to the explosion count for BCS P%+5 ; this ship (i.e. the number of vertices used as origins JMP EXL52 ; for explosion clouds), loop back to EXL52 to do a ; cloud for the next vertex PLA ; Restore the current random number seed to RAND+1 that STA RAND+1 ; we stored at the start of the routine LDA #%100 ; Call SETL1 to set the 6510 input/output port to the JSR SETL1 ; following: ; ; * LORAM = 0 ; * HIRAM = 0 ; * CHAREN = 1 ; ; This sets the entire 64K memory map to RAM ; ; See the memory map at the top of page 265 in the ; Programmer's Reference Guide LDA K%+6 ; Store the z_lo coordinate for the planet (which will STA RAND+3 ; be pretty random) in the RAND+3 seed RTS ; Return from the subroutine .EX112 JSR DORND2 ; Set A and X to random numbers, making sure the C flag ; doesn't affect the outcome JMP EX42 ; We just skipped a particle, so jump up to EX42 to do ; the next oneName: PTCLS2 [Show more] Type: Subroutine Category: Drawing ships Summary: Draw the explosion along with an explosion spriteContext: See this subroutine in context in the source code References: This subroutine is called as follows: * DOEXP calls PTCLS2
This routine is very similar to the PTCLS section of the DOEXP subroutine, except it draws an explosion sprite along with the explosion cloud. It is only called once for each explosion cloud, at the start of the explosion process.
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Label EX112 is local to this routine
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Label EX42 is local to this routine
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Label EXL22 is local to this routine
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Label EXL32 is local to this routine
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Label EXL42 is local to this routine
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Label EXL52 is local to this routine
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Workspace K% (category: Workspaces)
Ship data blocks and ship line heaps
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Subroutine PIXEL (category: Drawing pixels)
Draw a 1-pixel dot, 2-pixel dash or 4-pixel square
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Subroutine SETL1 (category: Utility routines)
Set the 6510 input/output port register to control the memory map
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Configuration variable VIC = $D000
Registers for the VIC-II video controller chip, which are memory-mapped to the 46 bytes from $D000 to $D02E (see page 454 of the Programmer's Reference Guide)
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Configuration variable Y = 72
The centre y-coordinate of the 256 x 144 space view
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Variable exlook (category: Drawing ships)
A table to shift X left by one place when X is 0 or 1
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Label noexpand is local to this routine
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Label yonk is local to this routine