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Elite on the BBC Micro

Version analysis of OUCH

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

Code variations between these versions are shown below.

Name: OUCH Type: Subroutine Category: Flight Summary: Potentially lose cargo or equipment following damage
Our shields are dead and we are taking damage, so there is a small chance of losing cargo or equipment.
.OUCH JSR DORND \ Set A and X to random numbers

Code variation 1 of 3A variation in the labels only

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Cassette, 6502SP, Master, Electron

Flight

BMI out \ If A < 0 (50% chance), return from the subroutine \ (as out contains an RTS) CPX #22 \ If X >= 22 (91% chance), return from the subroutine BCS out \ (as out contains an RTS) LDA QQ20,X \ If we do not have any of item QQ20+X, return from the BEQ out \ subroutine (as out contains an RTS). X is in the range \ 0-21, so this not only checks for cargo, but also for \ E.C.M., fuel scoops, energy bomb, energy unit and \ docking computer, all of which can be destroyed LDA DLY \ If there is already an in-flight message on-screen, BNE out \ return from the subroutine (as out contains an RTS)
BMI DK5 \ If A < 0 (50% chance), return from the subroutine \ (as DK5 contains an RTS) CPX #22 \ If X >= 22 (91% chance), return from the subroutine BCS DK5 \ (as DK5 contains an RTS) LDA QQ20,X \ If we do not have any of item QQ20+X, return from the BEQ DK5 \ subroutine (as DK5 contains an RTS). X is in the range \ 0-21, so this not only checks for cargo, but also for \ E.C.M., fuel scoops, energy bomb, energy unit and \ docking computer, all of which can be destroyed LDA DLY \ If there is already an in-flight message on-screen, BNE DK5 \ return from the subroutine (as DK5 contains an RTS)
 LDY #3                 \ Set bit 1 of de, the equipment destruction flag, so
 STY de                 \ that when we call MESS below, " DESTROYED" is appended
                        \ to the in-flight message

 STA QQ20,X             \ A is 0 (as we didn't branch with the BNE above), so
                        \ this sets QQ20+X to 0, which destroys any cargo or
                        \ equipment we have of that type

 CPX #17                \ If X >= 17 then we just lost a piece of equipment, so
 BCS ou1                \ jump to ou1 to print the relevant message

Code variation 2 of 3Minor and very low-impact

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Cassette, Flight, 6502SP, Electron

Master

TXA \ Print recursive token 48 + A as an in-flight token, ADC #208 \ which will be in the range 48 ("FOOD") to 64 ("ALIEN BNE MESS \ ITEMS") as the C flag is clear, so this prints the \ destroyed item's name, followed by " DESTROYED" (as we \ set bit 1 of the de flag above), and returns from the \ subroutine using a tail call
TXA \ Print recursive token 48 + A as an in-flight token, ADC #208 \ which will be in the range 48 ("FOOD") to 64 ("ALIEN JMP MESS \ ITEMS") as the C flag is clear, so this prints the \ destroyed item's name, followed by " DESTROYED" (as we \ set bit 1 of the de flag above), and returns from the \ subroutine using a tail call
.ou1

 BEQ ou2                \ If X = 17, jump to ou2 to print "E.C.M.SYSTEM
                        \ DESTROYED" and return from the subroutine using a tail
                        \ call

 CPX #18                \ If X = 18, jump to ou3 to print "FUEL SCOOPS
 BEQ ou3                \ DESTROYED" and return from the subroutine using a tail
                        \ call

 TXA                    \ Otherwise X is in the range 19 to 21 and the C flag is
 ADC #113-20            \ set (as we got here via a BCS to ou1), so we set A as
                        \ follows:
                        \
                        \   A = 113 - 20 + X + C
                        \     = 113 - 19 + X
                        \     = 113 to 115

Code variation 3 of 3Minor and very low-impact

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Cassette, Flight, Electron

6502SP, Master

BNE MESS \ Print recursive token A ("ENERGY BOMB", "ENERGY UNIT" \ or "DOCKING COMPUTERS") as an in-flight message, \ followed by " DESTROYED", and return from the \ subroutine using a tail call
JMP MESS \ Print recursive token A ("ENERGY BOMB", "ENERGY UNIT" \ or "DOCKING COMPUTERS") as an in-flight message, \ followed by " DESTROYED", and return from the \ subroutine using a tail call
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