The BBC and Master Computer Public Domain Library

A35D ASCNUM: Handle Exponential values and complete number conversion

Submitted by Steve Fewell

Routine:A35D> Name: ASCNUM: Handle Exponential value (decimal to binary conversion)
and complete the number conversion
Starting Address: &A35D

Description:

A35D:
Call A3BA to Extract Exponent value into A.
Add current exponent value ?&48 to the extracted exponent, and store back in &48.
Continue to A364 to complete the number conversion.

A364 ASCNUM: Complete the Number Conversion:

Store current PTRB pointer position back in &1B (PTRB Offset).
If exponent is zero and no decimal-point found then goto A399 (to copy
Integer to IWA and return with A=&40).

Gosub asign (if zero value in FWA then clear Sign, Exp, etc..., exit with A = sign)
if FWA = 0 then A395 [FWA = 0.0 so exit with C=1 and A=&FF].

Set FWA Exponent to 168 (Default value), FWA Overflow to 0 and FWA Sign to 0 (Positive value).
[Default exponent = 10101000 =&A8 = 2 to the power of 40 as offset from 128.]
gosub 81F7 to Normalise FWA#1 (correcting the exponent value).

If the ?&48 exponent is zero, then it doesn't need to be applied; otherwise:
If exponent in ?&48 is negative then A38B (keep dividing FWA by 10 and incrementing the
exponent by 1, until ?&48 is zero. [meaning that we have now applied the base 10 exponent].

If exponent in ?&48 is positive A382 (keep multiplying FWA by 10 and decrementing the
exponent by 1, until ?&48 is zero. [meaning that we have now applied the base 10 exponent].

After applying the exponent to the binary number, gosub A695 (round Mantissa to 4 bytes),
and exit with Carry=1 and A=&FF.

A399 ASCNUM: Copy Integer from FWA to IWA if it will fit:

If FWA Mantissa 1 is not 0 or FWA Mantissa byte 2's top bit is set (MSB) then the
integer value is too large to fit in the IWA; so go back to the Float routine.

Transfer Integer in FWA to IWA as follows:
    FWA Mantissa byte 2 ==> IWA Byte 4 (MSB)
    FWA Mantissa byte 3 ==> IWA Byte 3
    FWA Mantissa byte 4 ==> IWA Byte 2
    FWA Mantissa byte 5 (rounding) ==> IWA Byte 1 (LSB)

Exit with A=&40 and Carry = 1.

A3BA ASCNUM: Extract Exponent:

Get Next Character (after the 'E').
If '-' then get the Positive value (as described below) and then EOR with #&FF to
make it negative and exit with the carry set.

If '+' then get next character (skip the leading '+')

If first digit is invalid, return 0 as the exponend value (in A).
Subtract 48 to get the binary value of the digit - store in &49.

Get next character.
If second digit is invalid return with A = value in ?&49 [1 digit exponent].

Subtract 48 to get the binary value of the digit - store in &42.
Multiply the first digit by 10 (ASL, ASL, Add ?&49, ASL) and then add the second digit
to obtain the completed exponent.
Return with exponent in A, and carry set if exponent is negative; otherwise, with carry clear.

Disassembly for the ASCNUM: Handle Exponent Values & complete num conv routine