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BBC Micro Elite

Ship hanger: HALL (6502SP version)

Name: HALL [View in context] Type: Subroutine Category: Ship hanger Summary: Draw the ships in the ship hanger, then draw the hanger by sending an OSWORD 248 command to the I/O processor
Half the time this will draw one of the four pre-defined ship hanger groups in HATB, and half the time this will draw a solitary Sidewinder, Mamba, Krait or Adder on a random position. In all cases, the ships will be randomly spun around on the ground so they can face in any dirction, and larger ships are drawn higher up off the ground than smaller ships. The ships are drawn by the HAS1 routine, which uses the normal ship-drawing routine in LL9, and then the hanger background is drawn by sending an OSWORD 248 command to the I/O processor.
.HALL LDA #0 \ Send a #SETVDU19 0 command to the I/O processor to JSR DOVDU19 \ switch to the mode 1 palette for the space view, \ which is yellow (colour 1), red (colour 2) and cyan \ (colour 3) JSR UNWISE \ Call UNWISE, which does nothing in the 6502 Second \ Processor version of Elite (this routine does have a \ function in the disc version but isn't required here, \ so the authors presumably just cleared out the UNWISE \ routine rather than unplumbing it from the code) LDA #0 \ Clear the top part of the screen, draw a white border, JSR TT66 \ and set the current view type in QQ11 to 0 (space \ view) JSR DORND \ Set A and X to random numbers BPL HA7 \ Jump to HA7 if A is positive (50% chance) AND #3 \ Reduce A to a random number in the range 0-3 STA T \ Set X = A * 8 + A ASL A \ = 9 * A ASL A \ ASL A \ so X is a random number, either 0, 9, 18 or 27 ADC T TAX \ The following double loop calls the HAS1 routine three \ times to display three ships on screen. For each call, \ the values passed to HAS1 in XX15+2 to XX15 are taken \ from the HATB table, depending on the value in X, as \ follows: \ \ * If X = 0, pass bytes #0 to #2 of HATB to HAS1 \ then bytes #3 to #5 \ then bytes #6 to #8 \ \ * If X = 9, pass bytes #9 to #11 of HATB to HAS1 \ then bytes #12 to #14 \ then bytes #15 to #17 \ \ * If X = 18, pass bytes #18 to #20 of HATB to HAS1 \ then bytes #21 to #23 \ then bytes #24 to #26 \ \ * If X = 27, pass bytes #27 to #29 of HATB to HAS1 \ then bytes #30 to #32 \ then bytes #33 to #35 \ \ Note that the values are passed in reverse, so for the \ first call, for example, where we pass bytes #0 to #2 \ of HATB to HAS1, we call HAS1 with: \ \ XX15 = HATB+2 \ XX15+1 = HATB+1 \ XX15+2 = HATB LDY #3 \ Set CNT2 = 3 to act as an outer loop counter going STY CNT2 \ from 3 to 1, so the HAL8 loop is run 3 times .HAL8 LDY #2 \ Set Y = 2 to act as an inner loop counter going from \ 2 to 0 .HAL9 LDA HATB,X \ Copy the X-th byte of HATB to the Y-th byte of XX15, STA XX15,Y \ as described above INX \ Increment X to point to the next byte in HATB DEY \ Decrement Y to point to the previous byte in XX15 BPL HAL9 \ Loop back to copy the next byte until we have copied \ three of them (i.e. Y was 3 before the DEY) TXA \ Store X on the stack so we can retrieve it after the PHA \ call to HAS1 (as it contains the index of the next \ byte in HATB JSR HAS1 \ Call HAS1 to draw this ship in the hanger PLA \ Restore the value of X, so X points to the next byte TAX \ in HATB after the three bytes we copied into XX15 DEC CNT2 \ Decrement the outer loop counter in CNT2 BNE HAL8 \ Loop back to HAL8 to do it 3 times, once for each ship \ in the HATB table LDY #128 \ Set Y = 128 to send as byte #2 of the parameter block \ to the OSWORD 248 command below, to tell the I/O \ processor that there are multiple ships in the hanger BNE HA9 \ Jump to HA9 to display the ship hanger (this BNE is \ effectively a JMP as Y is never zero) .HA7 \ If we get here, A is a positive random number in the \ range 0-127 LSR A \ Set XX15+1 = A / 2 (random number 0-63) STA XX15+1 JSR DORND \ Set XX15 = random number 0-255 STA XX15 JSR DORND \ Set XX15+2 = #SH3 + random number 0-3 AND #3 \ ADC #SH3 \ which is the ship type of a Sidewinder, Mamba, Krait STA XX15+2 \ or Adder JSR HAS1 \ Call HAS1 to draw this ship in the hanger, with the \ the following properties: \ \ * Random x-coordinate from -63 to +63 \ \ * Randomly chosen Sidewinder, Mamba, Krait or Adder \ \ * Random z-coordinate from +256 to +639 LDY #0 \ Set Y = 0 to send as byte #2 of the parameter block to \ the OSWORD 248 command below, to tell the I/O \ processor that there is just one ship in the hanger .HA9 STY HANG+2 \ Store Y in byte #2 of the parameter block below JSR UNWISE \ Call UNWISE, which (as noted above) does nothing in \ the 6502 Second Processor version of Elite LDA #248 \ Set A in preparation for sending an OSWORD 248 command LDX #LO(HANG) \ Set (Y X) to point to the HANG parameter block LDY #HI(HANG) JMP OSWORD \ Send an OSWORD 248 command to the I/O processor to \ draw the ship hanger, returning from the subroutine \ using a tail call .HANG EQUB 3 \ The number of bytes to transmit with this command EQUB 0 \ The number of bytes to receive with this command EQUB 0 \ Multiple ship flag: \ \ * 0 = there is just one ship in the hanger \ \ * 128 = there are multiple ships in the hanger