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BBC Micro Elite

Flight: OOPS

Name: OOPS [View in context] 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 LDY #8 \ Fetch byte #8 (z_sign) for the ship attacking us, and LDX #0 \ 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 \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 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 \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 .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