ConvOP1_fast

;Attempts to convert the TI float in OP1 into an unsigned integer in HL.
;
;Throws:
; - A data type error if OP1 doesn't hold a nonnegative real number.
; - A domain error if the value cannot be exactly converted to an unsigned
;   16-bit integer.
;
;Don't try to hijack this routine with a pointer other than OP1. It won't work.
;
; I: (OP1)=float
; O: A=0, BC=?, DE=OP1+8, HL=(uint)(OP1)
;FO: S=0, Z=1, H=0, P/V=0, N=1, C=0
;CC: 69 + 154*((d+1)/2) + 131*(d/2) + (d%2 ? 21 : 12) + 43*(7-((d+1)/2)
;    d = (OP1)!=0 ? floor(log10((OP1))) + 1 : 1
ConvOP1:
;Throws an error if OP1 doesn't hold a nonnegative real number.
    ld  de,OP1
    ld  a,(de)
    or  a
    jr  nz,ErrDataType
;Initializes the 16-bit accumulator to 0 and reads the exponent.
    ld  h,a
    ld  l,a
    inc e
    ld  a,(de)
ConvOP1_Loop:
;Saves the exponent.
    push    af
;Reads the high nibble/digit of the next mantissa byte.
    inc e
    ld  a,(de)
;Multiplies the 16-bit accumulator by 10, checking for overflow where necessary,
;and shifts the high digit right by 4 bits into normal position.
    ld  b,h
    ld  c,l
    add hl,hl
    rra
    add hl,hl
    rra
    add hl,bc       ;9999*4=39996+9999=49995, cannot overflow yet
    rra
    add hl,hl       ;9999*5=49995*2=99990, can now overflow
    jr  c,ErrDomain
    rra
;Adds the now shifted high digit to the 16-bit accumulator, checking for
;overflow.
    ld  b,0
    ld  c,a
    add hl,bc
    jr  c,ErrDomain ;65530+9=65539, can still overflow
;Restores the exponent, decrements it, and breaks out if it becomes $7F. Doesn't
;care about bad exponents, as the 16-bit accumulator would overflow eventually.
    pop af
    dec a
    jp  pe,ConvOP1_DoneMidByte
;Multiplies the 16-bit accumulator by 10, checking for overflow where necessary.
    add hl,hl       ;40000*2=80000, can still overflow
    jr  c,ErrDomain
    ld  b,h
    ld  c,l
    add hl,hl       ;20000*2=40000*2=80000, can still overflow
    jr  c,ErrDomain
    add hl,hl       ;10000*4=40000*2=80000, can still overflow
    jr  c,ErrDomain
    add hl,bc       ;any 5-digit number has already overflowed
;Reads the low nibble/digit of the current mantissa byte and adds it to the
;16-bit accumulator.
    ld  b,a
    ld  a,(de)
    and $0F
    ld  c,a
    ld  a,b
    ld  b,0
    add hl,bc
;Decrements the exponent and continues looping if it doesn't become $7F.
    dec a
    jp  po,ConvOP1_Loop
;Skips conversion-finished-mid-byte code.
    jr  ConvOP1_CheckIntLoop
ConvOP1_DoneMidByte:
;Throws an error if the low nibble/digit of this mantissa byte is nonzero.
    ld  a,(de)
    and $0F
    jr  nz,ErrDomain
ConvOP1_CheckIntLoop:
;Returns successfully if the last byte of the mantissa has been checked.
#ifdef 83p
    or  e
    ret m
#else
    ld  a,(OP1+8)&$FF
    sub e
    ret z
#endif
;Continues if the next byte of the mantissa is zero.
    inc e
    ld  a,(de)
    or  a
    jr  z,ConvOP1_CheckIntLoop
ErrDomain:
;Throws a domain error.
    B_CALL(_ErrDomain)
ErrDataType:
;Throws a data type error.
    B_CALL(_ErrDataType)