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Elite on the BBC Micro

Version analysis of OOPS

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

Code variations between these versions are shown below.

Name: OOPS Type: Subroutine Category: Flight Summary: Take some damage
We just took some damage, so reduce the shields if we have any, or reduce the energy levels and potentially take some damage to the cargo if we don't. Arguments: A The amount of damage to take INF The address of the ship block for the ship that attacked us, or the ship that we just ran into
.OOPS STA T \ Store the amount of damage in T

Code variation 1 of 3Minor and very low-impact

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Cassette, Electron

Flight, 6502SP, Master

LDY #8 \ Fetch byte #8 (z_sign) for the ship attacking us, and LDX #0 \ set X = 0 LDA (INF),Y
LDX #0 \ Fetch byte #8 (z_sign) for the ship attacking us, and LDY #8 \ set X = 0 LDA (INF),Y
 BMI OO1                \ If A is negative, then we got hit in the rear, so jump
                        \ to OO1 to process damage to the aft shield

 LDA FSH                \ Otherwise the forward shield was damaged, so fetch the
 SBC T                  \ shield strength from FSH and subtract the damage in T

 BCC OO2                \ If the C flag is clear then this amount of damage was
                        \ too much for the shields, so jump to OO2 to set the
                        \ shield level to 0 and start taking damage directly
                        \ from the energy banks

 STA FSH                \ Store the new value of the forward shield in FSH

 RTS                    \ Return from the subroutine

.OO2

Code variation 2 of 3Minor and very low-impact

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Cassette

Flight, Electron

Master

6502SP

\LDX #0 \ This instruction is commented out in the original \ source, and isn't required as X is set to 0 above STX FSH \ Set the forward shield to 0
STX FSH \ Set the forward shield to 0
LDX #0 \ Set the forward shield to 0 STX FSH
STZ FSH \ Set the forward shield to 0
 BCC OO3                \ Jump to OO3 to start taking damage directly from the
                        \ energy banks (this BCC is effectively a JMP as the C
                        \ flag is clear, as we jumped to OO2 with a BCC)

.OO1

 LDA ASH                \ The aft shield was damaged, so fetch the shield
 SBC T                  \ strength from ASH and subtract the damage in T

 BCC OO5                \ If the C flag is clear then this amount of damage was
                        \ too much for the shields, so jump to OO5 to set the
                        \ shield level to 0 and start taking damage directly
                        \ from the energy banks

 STA ASH                \ Store the new value of the aft shield in ASH

 RTS                    \ Return from the subroutine

.OO5

Code variation 3 of 3Minor and very low-impact

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

Cassette

Flight, Electron

Master

6502SP

\LDX #0 \ This instruction is commented out in the original \ source, and isn't required as X is set to 0 above STX ASH \ Set the aft shield to 0
STX ASH \ Set the aft shield to 0
LDX #0 \ Set the forward shield to 0 STX ASH
STZ ASH \ Set the aft shield to 0
.OO3

 ADC ENERGY             \ A is negative and contains the amount by which the
 STA ENERGY             \ damage overwhelmed the shields, so this drains the
                        \ energy banks by that amount (and because the energy
                        \ banks are shown over four indicators rather than one,
                        \ but with the same value range of 0-255, energy will
                        \ appear to drain away four times faster than the
                        \ shields did)

 BEQ P%+4               \ If we have just run out of energy, skip the next
                        \ instruction to jump straight to our death

 BCS P%+5               \ If the C flag is set, then subtracting the damage from
                        \ the energy banks didn't underflow, so we had enough
                        \ energy to survive, and we can skip the next
                        \ instruction to make a sound and take some damage

 JMP DEATH              \ Otherwise our energy levels are either 0 or negative,
                        \ and in either case that means we jump to our DEATH,
                        \ returning from the subroutine using a tail call

 JSR EXNO3              \ We didn't die, so call EXNO3 to make the sound of a
                        \ collision

 JMP OUCH               \ And jump to OUCH to take damage and return from the
                        \ subroutine using a tail call

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