GN_Gdn, convert an ASCII string to a binary integer number

RST 20H, DEFW $1009

IN:

     HL = local source pointer
          HL = 0, read from stream IX handle
          HL = 1, read from filter IX handle
          HL = 2, not allowed
          > 255, read string at (HL)
     DE = destination pointer.
          DE = 0, 1 not allowed
          DE = 2, return integer in BC (converted to 16bit
          range)
          > 255, store integer at (DE). (DE) = least
          significant byte, (DE+3)= most significant byte.

     B = maximum number of characters to be read
     IX = source handle (if HL = 0,1)

     Fc = 0
     Fz = 1, conversion recognised a number
     A = character which stopped conversion
     HL, if HL(in) < 2, indexes conversion terminator
     HL, if HL(in) > 255, then points to conversion terminator.
     BC = result, if DE(in) = 2

     Fc = 1
     A = error code:
          RC_OVF ($0B), overflow, result invalid
          RC_EOF ($09), reading an empty stream or filter

     ...DE../IXIY same
     AFB..HL/.... different

Parsing will stop at the first non-decimal digit. If the source text was numeric by not supplied with a terminator, the routine will set Fz = 1 and exit with A=0 and Fc = 0.

This call will fail if the source or destination pointers are in segment 2.

GN_D16, 16bit unsigned division
GN_M16, 16bit unsigned multiplication
GN_D24, 24bit unsigned division
GN_M24, 24bit unsigned multiplication
GN_Pdn, write number as decimal ASCII string