Routines
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AC75: Divide
Used by the routines at Print_Completion_Percentage and Set_Up_Line.
Divides H multiplied by 0100 by DE, one bit at a time. The quotient has eight bits, so DE must be greater than H for the answer to be right. The quotient is built up in the shadow registers and the working is done in the main registers, so the shadow registers are not preserved.
The line drawing code uses it to find how far to move across for each pixel moved down.
Input
H Dividend (it is multiplied by 0100)
DE Divisor
Output
HL Quotient
DE Remainder
Divide AC75 LD L,H Copy the dividend to L.
AC76 LD H,$00 Clear H.
AC78 EXX Switch to the shadow registers.
AC79 LD HL,$0000 HL'=00 (the quotient so far).
AC7C LD B,$08 B'=08 (the number of bits).
Divide_Loop AC7E EXX Switch to the main registers.
AC7F SLA L Shift the dividend left.
AC81 RL H
AC83 PUSH HL
AC84 AND A
AC85 SBC HL,DE Try to subtract the divisor.
AC87 JR C,Divide_Restore Jump to Divide_Restore if it does not go.
AC89 POP AF Discard the stashed dividend.
AC8A EXX Switch to the shadow registers.
AC8B ADD HL,HL Shift the quotient left.
AC8C INC HL Add one for the bit.
Divide_Next AC8D DJNZ Divide_Loop Decrease the bit counter and loop back until all eight bits have been done.
AC8F PUSH HL Stash the quotient on the stack.
AC90 EXX Switch to the main registers.
AC91 LD E,L Copy the remainder to DE.
AC92 LD D,H Copy H to D.
AC93 POP HL Restore HL from the stack.
AC94 RET Restore the quotient from the stack.
Divide_Restore AC95 POP HL Restore the stashed dividend (the subtraction is undone).
AC96 EXX Switch to the shadow registers.
AC97 ADD HL,HL Shift the quotient left.
AC98 JR Divide_Next Jump to Divide_Next.
View the equivalent code in;
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