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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 30A9 PUSH BC BC is saved.
30AA LD B,$10 It is to be a 16-bit multiplication.
30AC LD A,H A holds the high byte.
30AD LD C,L C holds the low byte.
30AE LD HL,$0000 Initialise the result to zero.
HL_LOOP 30B1 ADD HL,HL Double the result.
30B2 JR C,HL_END Jump if overflow.
30B4 RL C Rotate bit 7 of C into the carry.
30B6 RLA Rotate the carry bit into bit 0 and bit 7 into the carry flag.
30B7 JR NC,HL_AGAIN Jump if the carry flag is reset.
30B9 ADD HL,DE Otherwise add DE in once.
30BA JR C,HL_END Jump if overflow.
HL_AGAIN 30BC DJNZ HL_LOOP Repeat until 16 passes have been made.
HL_END 30BE POP BC Restore BC.
30BF RET Finished.
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