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This subroutine is called by GET_HLxDE and by multiply to perform the 16-bit multiplication as stated.
Any overflow of the 16 bits available is dealt with on return from the subroutine.
DE First number (M)
HL Second number (N)
HL_HLxDE 12457 PUSH BC BC is saved.
12458 LD B,16 It is to be a 16-bit multiplication.
12460 LD A,H A holds the high byte.
12461 LD C,L C holds the low byte.
12462 LD HL,0 Initialise the result to zero.
HL_LOOP 12465 ADD HL,HL Double the result.
12466 JR C,HL_END Jump if overflow.
12468 RL C Rotate bit 7 of C into the carry.
12470 RLA Rotate the carry bit into bit 0 and bit 7 into the carry flag.
12471 JR NC,HL_AGAIN Jump if the carry flag is reset.
12473 ADD HL,DE Otherwise add DE in once.
12474 JR C,HL_END Jump if overflow.
HL_AGAIN 12476 DJNZ HL_LOOP Repeat until 16 passes have been made.
HL_END 12478 POP BC Restore BC.
12479 RET Finished.
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