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Elite on the BBC Micro

Version analysis of MAS2

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

Code variations between these versions are shown below.

Name: MAS2 Type: Subroutine Category: Maths (Geometry)

Code variation 1 of 1A variation in the comments only

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Cassette, Flight, 6502SP, Master


Summary: Calculate a cap on the maximum distance to the planet or sun
Summary: Calculate a cap on the maximum distance to the planet

Given a value in Y that points to the start of a ship data block as an offset from K%, calculate the following: A = A OR x_sign OR y_sign OR z_sign and clear the sign bit of the result. The K% workspace contains the ship data blocks, so the offset in Y must be 0 or a multiple of NI% (as each block in K% contains NI% bytes). The result effectively contains a maximum cap of the three values (though it might not be one of the three input values - it's just guaranteed to be larger than all of them). If Y = 0 and A = 0, then this calculates the maximum cap of the highest byte containing the distance to the planet, as K%+2 = x_sign, K%+5 = y_sign and K%+8 = z_sign (the first slot in the K% workspace represents the planet). Arguments: Y The offset from K% for the start of the ship data block to use Returns: A A OR K%+2+Y OR K%+5+Y OR K%+8+Y, with bit 7 cleared Other entry points: m Do not include A in the calculation
.m LDA #0 \ Set A = 0 and fall through into MAS2 to calculate the \ OR of the three bytes at K%+2+Y, K%+5+Y and K%+8+Y .MAS2 ORA K%+2,Y \ Set A = A OR x_sign OR y_sign OR z_sign ORA K%+5,Y ORA K%+8,Y AND #%01111111 \ Clear bit 7 in A RTS \ Return from the subroutine
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