Routines
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CF7E: Multiply
Used by the routine at ScaleDisplacement.
Multiplies the byte in D by the byte in C.
Input
D One byte
C The other byte
Output
HL The product
The byte in D is multiplied by the byte in C. Eight bits are tested, and each bit that is set adds the shifted byte into the product.
Multiply CF7E LD H,$00 The product starts at zero, with eight bits to test.
CF80 LD B,H
CF81 LD L,H
CF82 LD E,D
CF83 LD A,$08
CF85 JR Multiply_Bit Test the first bit.
Multiply_Loop CF87 DEC A One fewer bit remains.
CF88 RET Z The product is ready.
CF89 SLA C Shift the other byte up.
CF8B RL B
Multiply_Bit CF8D RR D The next bit of the byte.
CF8F JR NC,Multiply_Loop This bit adds nothing.
CF91 ADD HL,BC Add the shifted byte into the product.
CF92 JP Multiply_Loop The next bit.
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