The BBC and Master Computer Public Domain Library

A2E1 ASCNUM: Extract Number at PTRB

Submitted by Steve Fewell

Routine:ASCNUM:ExtractNum
Name: ASCII TO NUM: Extract Integer/Floating-Point Number at PTRB
Starting Address: &A2E1
Entry criteria: The SWA contains the ASCII number to extract
Exit: Depending of the size of the number, and whether it contains a fractional part
 or not, either the FWA or IWA will contain the binary conversion
or the number. A=&40 if Integer, or &FF if Floating-point.
Carry is clear if the ASCII text could not be converted to a numeric value.

Description:

Clears bytes &31 to &35 (The FWA Mantissa & rounding bytes).
Clears byte &47 - the 'Decimal Point found' flag. (remains zero if number is Integer)
Clears byte &48 - the Exponent of the extracted number. (remains zero if number is Integer)

If Next Character at PTRB is "." then A31C [if decimal point has already been found,
      then A364 (Complete & tidy-up the number value) as the number is complete.
      Otherwise, increment &47 (to set the decimal point found flag)
      then goto A315 (to process the next character of the number).]

If Next Character at PTRB >= ":" or <= "/" then A2DA [Return 0.00 as no number found.].
Subtract 48 (47 + 1 as Carry Clear), so A=0 for "0", 1 for "1" ..., 9 for "9".
Store digit found at LSB of FWA (&35, the FWA rounding byte).

Load next character from PTRB in A.
If character >= ":" [non-digit] then A324 [process main body of number], as this checks
for "E" before terminating number extraction at an invalid character.]
Subtract 48 (47 + 1 as Carry Clear), so A=0 for "0", 1 for "1" ..., 9 for "9".
If carry clear (underflow occurred, A <= "/" [non-digit]) then A316 [ if char not = "." then A324
      (to process main body of number); otherwise, If character is "." then A31C [if decimal point has already been
      found, then A364 (Complete & tidy-up the number value) as the number is complete.
      Otherwise, increment &47 (to set the decimal point found flag)
      then goto A315 (to process the next character of the number).] ]

Temporarily store A in &2E [FWA sign].
Multiplies the number in &35 by 10 and adds ?&2E to the result to form
a 2-digit binary number.
The multiply does this: ?&35=((?&35*4)+?&35)*2) + ?&2E
= ?&35=(?&35*5*2) plus ?&2E.

Get next character, and if it isn't "." then goto A324 [to process the main body of the
number]. Otherwise, set decimal found flag and get next character before going to A324 [main body]
or finish extraction (A364) if the next character was another ".".

ASCNUM: A324: Process main body of number:
This routine is first entered after the first two digits have been processed, (by A2E1) or after a "."
if the decimal point was encountered before that stage. This routine doesn't need to deal
with a "no number found" situation, as this has already been dealt with in A2E1.
This routine keeps the Exponent &48 in line with the binary value, and copes with
exponential values ("E"). It also copes with much bigger numbers than the 2-digit ones that
A2E1 processed.

This routines does the following: If ASCII character is "E" then goto &A35D [to get the exponent value and convert the exponential
number to the completed binary value and exit with the result stored in the FWA]
If the character is not a valid digit (char >= ":" or char <= "/") then goto A364 to complete
the number and finish the extraction.
Subtract 48 (47 + 1 as Carry Clear), so A=0 for "0", 1 for "1" ..., 9 for "9".

X = ?&31 (FWA Mantissa byte 1). If this value is less than or equal to 24 then it is
ok to add the next digit to the end of the number [so goto &A33E]. Otherwise, the digit is not
added [number is too large] and the exponent is incremented (if no decimal point has been
found [integer value]). Then goto A315. A315 gets the next character, handles any decimal point
character found, or calls A324 again to handle all other characters.

ASCNUM: A33E: Add digit to end of number (and multiply 'old' number by 10):
If the number has a decimal point in it (?&47 not = 0) then decrement &48 (the exponent)
as we are adding a digit to the LSB of the value without moving the decimal point from its current position.

Multiply the current number by 10 (using &A279 [multiply FWA Mantissa by 10] routine).
&A279 returns with A unchanged, so we can add the next digit to &35 in order to
add the new digit to the end of the current number (now multiplied by 10 to cope with decimal-binary conversion).
If overflow occures, then we need to increment the rest of the bytes of the FWA Mantissa
(&34 and then &33 (if incrementing &34 overflowed), etc... to &31 if necessary).
lastly, we need to go back to A315 to get the next charater.
&A315 gets the next character, handles any decimal point character found, or calls &A324 again to
handle all other characters.

Disassembly for the ASCNUM: Extract Number at PTRB routine