Technically speaking, the TI99/4a uses a 7-digit Radix-100 mantissa for internal calculations.
A single Radix-100 digit has a range of value from 0 to 99 in base-10 arithmetic.
This means that a 7-digit Radix-100 number will correspond to decimal precision of 13 to 14 digits, depending on the value.
Radix-100 exponents range in value from -64 to +63 which yield decimal values of 10-128 to 10+126.
The Radix-100 mantissa and exponent combine to provide an equivalent decimal range of from -9.9999999999999E127 through -1.0000000000000E-128; zero; and then +1.0000000000000E-128 on through +9.9999999999999E127.
The internal format of each numerical value consists of eight bytes. The first byte contains the exponent and its sign, biased by 40 hex. The remaining bytes contain the mantissa, with the most significant digit first.
The number is normalized so that the decimal point is immediately after the most significant digit.
If the number is negative, then the first two bytes are complemented.
1. The number 12710 is represented as: EXP MSD LSD 41 01 1B 00 00 00 00 00
2. The fraction 0.510 is represented as: 3F 32 00 00 00 00 00 00
3a. The value of pi/2 is represented as: 40 01 39 07 60 20 43 5F
3b. The value of -pi/2 is: BF FF 39 07 60 20 43 5F
(extracted from Users Reference Guide)