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Elite on the BBC Micro

Version analysis of Main flight loop (Part 8 of 16)

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

Code variations between these versions are shown below.

Name: Main flight loop (Part 8 of 16) Type: Subroutine Category: Main loop Summary: For each nearby ship: Process us potentially scooping this item Deep dive: Program flow of the main game loop
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * Process us potentially scooping this item

Code variation 1 of 4Related to an enhanced feature

In the enhanced versions, the top nibble of the first byte of each ship blueprint defines whether a ship is scoopable, and the type of item that we get when we scoop it (so scooping Thargons gives alien items, and so on). The cassette version implements this functionality using hard-coded conditional statements, while the enhanced versions support more flexibility by using the ship data block to determine scoopability.

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Cassette

Flight, 6502SP, Master

Electron

LDA #3 \ Set A to 3 to denote we may be scooping an escape pod CPX #TGL \ If ship type < Thargon, i.e. it's a canister, jump BCC oily \ to oily to randomly decide the canister's contents BNE slvy2 \ If ship type <> Thargon, i.e. it's an escape pod, \ jump to slvy2 with A set to 3, so we scoop up the \ escape pod as slaves LDA #16 \ Otherwise this is a Thargon, so jump to slvy2 with BNE slvy2 \ A set to 16, so we scoop up the Thargon as alien items \ (this BNE is effectively a JMP as A will never be \ zero)
CPX #OIL \ If this is a cargo canister, jump to oily to randomly BEQ oily \ decide the canister's contents LDY #0 \ Fetch byte #0 of the ship's blueprint LDA (XX0),Y LSR A \ Shift it right four times, so A now contains the high LSR A \ nibble (i.e. bits 4-7) LSR A LSR A BEQ MA58 \ If A = 0, jump to MA58 to skip all the docking and \ scooping checks \ Only the Thargon, alloy plate, splinter and escape pod \ have non-zero upper nibbles in their blueprint byte #0 \ so if we get here, our ship is one of those, and the \ upper nibble gives the market item number of the item \ when scooped, less 1 ADC #1 \ Add 1 to the upper nibble to get the market item \ number BNE slvy2 \ Skip to slvy2 so we scoop the ship as a market item
LDA #3 \ Set A to 3 to denote we may be scooping an escape pod CPX #ESC \ If this is not an escape pod, jump to oily to randomly BNE oily \ decide the canister's contents BEQ slvy2 \ This is an escape pod, so jump to slvy2 with A set to \ 3, so we scoop up the escape pod as slaves
.oily

 JSR DORND              \ Set A and X to random numbers and reduce A to a
 AND #7                 \ random number in the range 0-7

.slvy2

Code variation 2 of 4A variation in the comments only

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Cassette, Flight, 6502SP, Master

Electron

\ By the time we get here, we are scooping, and A \ contains the type of item we are scooping (a random \ number 0-7 if we are scooping a cargo canister, 3 if \ we are scooping an escape pod, or 16 if we are \ scooping a Thargon). These numbers correspond to the \ relevant market items (see QQ23 for a list), so a \ cargo canister can contain anything from food to \ computers, while escape pods contain slaves, and \ Thargons become alien items when scooped
\ By the time we get here, we are scooping, and A \ contains the type of item we are scooping (a random \ number 0-7 if we are scooping a cargo canister, or 3 \ if we are scooping an escape pod). These numbers \ correspond to the relevant market items (see QQ23 \ for a list), so a cargo canister can contain \ anything from food to computers, while escape pods \ contain slaves

Code variation 3 of 4Specific to an individual platform

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Cassette, Electron

Flight, 6502SP, Master

STA QQ29 \ Call tnpr with the scooped cargo type stored in QQ29 LDA #1 \ and A set to 1, to work out whether we have room in JSR tnpr \ the hold for the scooped item (A is preserved by this \ call, and the C flag contains the result)
JSR tnpr1 \ Call tnpr1 with the scooped cargo type stored in A \ to work out whether we have room in the hold for one \ tonne of this cargo (A is set to 1 by this call, and \ the C flag contains the result)
 LDY #78                \ This instruction has no effect, so presumably it used
                        \ to do something, but didn't get removed

 BCS MA59               \ If the C flag is set then we have no room in the hold
                        \ for the scooped item, so jump down to MA59 make a
                        \ sound to indicate failure, before destroying the
                        \ canister

 LDY QQ29               \ Scooping was successful, so set Y to the type of
                        \ item we just scooped, which we stored in QQ29 above

 ADC QQ20,Y             \ Add A (which we set to 1 above) to the number of items
 STA QQ20,Y             \ of type Y in the cargo hold, as we just successfully
                        \ scooped one canister of type Y

 TYA                    \ Print recursive token 48 + Y as an in-flight token,
 ADC #208               \ which will be in the range 48 ("FOOD") to 64 ("ALIEN
 JSR MESS               \ ITEMS"), so this prints the scooped item's name

Code variation 4 of 4Related to an enhanced feature

In the enhanced version there is a difference between a ship that has been killed and a ship that has docked or been scooped, unlike in the cassette version where they are the same thing.

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Cassette, Electron

Flight, 6502SP, Master

JMP MA60 \ We are done scooping, so jump down to MA60 to set the \ kill flag on the canister, as it no longer exists in \ the local bubble
ASL NEWB \ The item has now been scooped, so set bit 7 of its SEC \ NEWB flags to indicate this ROR NEWB
.MA65

 JMP MA26               \ If we get here, then the ship we are processing was
                        \ too far away to be scooped, docked or collided with,
                        \ so jump to MA26 to skip over the collision routines
                        \ and move on to missile targeting

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