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Elite on the BBC Micro

Version analysis of DOEXP

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

Name: DOEXP Type: Subroutine Category: Drawing ships Summary: Draw an exploding ship Deep dive: Drawing explosion clouds Generating random numbers
.EX2 LDA INWK+31 \ Set bits 5 and 7 of the ship's byte #31 to denote that ORA #%10100000 \ the ship is exploding and has been killed STA INWK+31 RTS \ Return from the subroutine .DOEXP

Code variation 1 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

LDA INWK+31 \ If bit 6 of the ship's byte #31 is clear, then the AND #%01000000 \ ship is not already exploding so there is no existing BEQ P%+5 \ explosion cloud to remove, so skip the following \ instruction JSR PTCLS \ Call PTCLS to remove the existing cloud by drawing it \ again LDA INWK+6 \ Set T = z_lo STA T LDA INWK+7 \ Set A = z_hi, so (A T) = z CMP #32 \ If z_hi < 32, skip the next two instructions BCC P%+6 LDA #&FE \ Set A = 254 and jump to yy (this BNE is effectively a BNE yy \ JMP, as A is never zero) ASL T \ Shift (A T) left twice ROL A ASL T ROL A SEC \ And then shift A left once more, inserting a 1 into ROL A \ bit 0 \ Overall, the above multiplies A by 8 and makes sure it \ is at least 1, to leave a one-byte distance in A. We \ can use this as the distance for our cloud, to ensure \ that the explosion cloud is visible even for ships \ that blow up a long way away .yy STA Q \ Store the distance to the explosion in Q LDY #1 \ Fetch byte #1 of the ship line heap, which contains LDA (XX19),Y \ the cloud counter

Code variation 2 of 23Specific to an individual platform

This variation is blank in the Cassette, Disc (flight), Disc (docked), 6502 Second Processor and Electron versions.

Master

STA CLCNT \ Store the cloud counter in CLCNT (though this value is \ never read, so this has no effect)

Code variation 3 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

ADC #4 \ Add 4 to the cloud counter, so it ticks onwards every \ we redraw it BCS EX2 \ If the addition overflowed, jump up to EX2 to update \ the explosion flags and return from the subroutine STA (XX19),Y \ Store the updated cloud counter in byte #1 of the ship \ line heap JSR DVID4 \ Calculate the following: \ \ (P R) = 256 * A / Q \ = 256 * cloud counter / distance \ \ We are going to use this as our cloud size, so the \ further away the cloud, the smaller it is, and as the \ cloud counter ticks onward, the cloud expands LDA P \ Set A = P, so we now have: \ \ (A R) = 256 * cloud counter / distance CMP #&1C \ If A < 28, skip the next two instructions BCC P%+6 LDA #&FE \ Set A = 254 and skip the following (this BNE is BNE LABEL_1 \ effectively a JMP as A is never zero) ASL R \ Shift (A R) left three times to multiply by 8 ROL A ASL R ROL A ASL R ROL A \ Overall, the above multiplies (A R) by 8 to leave a \ one-byte cloud size in A, given by the following: \ \ A = 8 * cloud counter / distance .LABEL_1

Code variation 4 of 23A variation in the comments only

This variation is blank in the Cassette, Disc (flight), Disc (docked), Master and Electron versions.

6502SP

\ In the 6502 Second Processor version, the LABEL_1 \ label is actually `_ (a backtick followed by an \ underscore), but that doesn't compile in BeebAsm and \ it's pretty cryptic, so instead this version sticks \ with the label LABEL_1 from the cassette version

Code variation 5 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

DEY \ Decrement Y to 0 STA (XX19),Y \ Store the cloud size in byte #0 of the ship line heap LDA INWK+31 \ Clear bit 6 of the ship's byte #31 to denote that the AND #%10111111 \ explosion has not yet been drawn STA INWK+31 AND #%00001000 \ If bit 3 of the ship's byte #31 is clear, then nothing BEQ TT48 \ is being drawn on-screen for this ship anyway, so \ return from the subroutine (as TT48 contains an RTS) LDY #2 \ Otherwise it's time to draw an explosion cloud, so LDA (XX19),Y \ fetch byte #2 of the ship line heap into Y, which we TAY \ set to the explosion count for this ship (i.e. the \ number of vertices used as origins for explosion \ clouds) \ \ The explosion count is stored as 4 * n + 6, where n is \ the number of vertices, so the following loop copies \ the coordinates of the first n vertices from the heap \ at XX3, which is where we stored all the visible \ vertex coordinates in part 8 of the LL9 routine, and \ sticks them in the ship line heap pointed to by XX19, \ starting at byte #7 (so it leaves the first 6 bytes of \ the ship line heap alone) .EXL1 LDA XX3-7,Y \ Copy byte Y-7 from the XX3 heap, into the Y-th byte of STA (XX19),Y \ the ship line heap DEY \ Decrement the loop counter CPY #6 \ Keep copying vertex coordinates into the ship line BNE EXL1 \ heap until Y = 6 (which will copy n vertices, where n \ is the number of vertices we should be exploding) LDA INWK+31 \ Set bit 6 of the ship's byte #31 to denote that the ORA #%01000000 \ explosion has been drawn (as it's about to be) STA INWK+31 .PTCLS \ 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

Code variation 6 of 23Related to the Master version

The Master and Electron versions have half the number of explosion particles per vertex compared to the other versions.

This variation is blank in the Cassette, Disc (flight), Disc (docked) and Electron versions.

Tap on a version to expand it, and tap it again to show to all variations.

Master

6502SP

LSR A \ Divide A by 16 so that is has a maximum value of 7 LSR A LSR A LSR A
LSR A \ Divide A by 8 so that is has a maximum value of 15 LSR A LSR A

Code variation 7 of 23Related to the Electron version

The Master and Electron versions have half the number of explosion particles per vertex compared to the other versions.

This variation is blank in the Disc (docked), 6502 Second Processor and Master versions.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight

Electron

LSR A \ Divide A by 8 so that is has a maximum value of 15 LSR A LSR A
LSR A \ Divide A by 16 so that is has a maximum value of 7 LSR A LSR A LSR A

Code variation 8 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

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) .EXL5 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 .EXL3 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 EXL3 \ Loop back to EXL3 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 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 .EXL2 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

Code variation 9 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, 6502SP, Electron

Master

STA &FFFD,Y \ Y is going from 3 to 6, so this stores the four bytes \ in memory locations &00, &01, &02 and &03, which are \ the memory locations of RAND through RAND+3
IF _SNG47 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 ELIF _COMPACT STA &0000,Y \ Y is going from 3 to 6, so this stores the four bytes \ in memory locations &03, &04, &05 and &06, which are \ the memory locations of RAND through RAND+3 ENDIF

Code variation 10 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

CPY #6 \ Loop back to EXL2 until Y = 6, which means we have BNE EXL2 \ 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

Code variation 11 of 23Specific to an individual platform

This variation is blank in the Cassette, Disc (flight), Disc (docked), 6502 Second Processor and Electron versions.

Master

STY CNT2 \ Store the number of explosion particles in CNT2, to \ use this as a loop counter to iterate through all the \ particles in the explosion

Code variation 12 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

.EXL4

Code variation 13 of 23Minor and very low-impact

This variation is blank in the Disc (docked) version.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, 6502SP, Electron

Master

JSR DORND2 \ Set ZZ to a random number, making sure the C flag STA ZZ \ doesn't affect the outcome
CLC \ This contains the code from the DORND2 routine, so LDA RAND \ this section is exactly equivalent to a JSR DORND2 ROL A \ call, but is slightly faster as it's been inlined TAX \ (so it sets A and X to random values, making sure ADC RAND+2 \ the C flag doesn't affect the outcome) STA RAND STX RAND+2 LDA RAND+1 TAX ADC RAND+3 STA RAND+1 STX RAND+3 STA ZZ \ Set ZZ to a random number

Code variation 14 of 23Related to the Master version

In the Master version, explosions are made up of yellow, red and cyan particles.

This variation is blank in the Cassette, Disc (flight), Disc (docked), 6502 Second Processor and Electron versions.

Master

AND #3 \ Set X to this random number, reduced to be in the TAX \ range 0-3 LDA EXCOL,X \ Set the colour randomly to one of the four colours STA COL \ in the EXCOL table, so explosions are made up of \ yellow, red and cyan particles

Code variation 15 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

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 EX11 \ If A is non-zero, the particle is off-screen as the \ coordinate is bigger than 255), so jump to EX11 to do \ the next particle CPX #2*Y-1 \ If X > the y-coordinate of the bottom of the screen, BCS EX11 \ the particle is off the bottom of the screen, so jump \ to EX11 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 EX4 \ If A is non-zero, the particle is off-screen as the \ coordinate is bigger than 255), so jump to EX11 to do \ the next particle \ Otherwise X contains a random x-coordinate within the \ cloud LDA Y1 \ Set A = our random y-coordinate within the cloud

Code variation 16 of 23Minor and very low-impact

This variation is blank in the Disc (docked) version.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Master, Electron

6502SP

JSR PIXEL \ Draw a point at screen coordinate (X, A) with the \ point size determined by the distance in ZZ
JSR PIXEL3 \ Draw a point at screen coordinate (X, A) with the \ point size determined by the distance in ZZ

Code variation 17 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, 6502SP, Electron

Master

.EX4 DEY \ Decrement the loop counter for the next particle
.EX4 DEC CNT2 \ Decrement the loop counter in CNT2 for the next \ particle

Code variation 18 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

BPL EXL4 \ Loop back to EXL4 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 BCC EXL5 \ this ship (i.e. the number of vertices used as origins \ for explosion clouds), loop back to EXL5 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

Code variation 19 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

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 .EX11

Code variation 20 of 23Minor and very low-impact

This variation is blank in the Disc (docked) version.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, 6502SP, Electron

Master

JSR DORND2 \ Set A and X to random numbers, making sure the C flag \ doesn't affect the outcome
CLC \ This contains the code from the DORND2 routine, so LDA RAND \ this section is exactly equivalent to a JSR DORND2 ROL A \ call, but is slightly faster as it's been inlined TAX \ (so it sets A and X to random values, making sure ADC RAND+2 \ the C flag doesn't affect the outcome) STA RAND STX RAND+2 LDA RAND+1 TAX ADC RAND+3 STA RAND+1 STX RAND+3

Code variation 21 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

JMP EX4 \ We just skipped a particle, so jump up to EX4 to do \ the next one .EXS1 \ This routine calculates the following: \ \ (A X) = (A R) +/- random * cloud size \ \ returning with the flags set for the high byte in A STA S \ Store A in S so we can use it later

Code variation 22 of 23Minor and very low-impact

This variation is blank in the Disc (docked) version.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, 6502SP, Electron

Master

JSR DORND2 \ Set A and X to random numbers, making sure the C flag \ doesn't affect the outcome
CLC \ This contains the code from the DORND2 routine, so LDA RAND \ this section is exactly equivalent to a JSR DORND2 ROL A \ call, but is slightly faster as it's been inlined TAX \ (so it sets A and X to random values, making sure ADC RAND+2 \ the C flag doesn't affect the outcome) STA RAND STX RAND+2 LDA RAND+1 TAX ADC RAND+3 STA RAND+1 STX RAND+3

Code variation 23 of 23Specific to an individual platform

This variation is blank in the Disc (docked) version.

Cassette, Flight, 6502SP, Master, Electron

ROL A \ Set A = A * 2 BCS EX5 \ If bit 7 of A was set (50% chance), jump to EX5 JSR FMLTU \ Set A = A * Q / 256 \ = random << 1 * projected cloud size / 256 ADC R \ Set (A X) = (S R) + A TAX \ = (S R) + random * projected cloud size \ \ where S contains the argument A, starting with the low \ bytes LDA S \ And then the high bytes ADC #0 RTS \ Return from the subroutine .EX5 JSR FMLTU \ Set T = A * Q / 256 STA T \ = random << 1 * projected cloud size / 256 LDA R \ Set (A X) = (S R) - T SBC T \ TAX \ where S contains the argument A, starting with the low \ bytes LDA S \ And then the high bytes SBC #0
 RTS                    \ Return from the subroutine

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