FAC
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The FAC is the floating-point accumulator, located in C64 memory at 97–102 ($61–$66). It stores floating-point numbers as they are being processed by the C64 BASIC interpreter.
The FAC is designed to handle 40-bit floating-point numbers, so allowing the C64 to perform relatively high precision floating-point arithmetic for its time. This 40-bit format is broken down into 8 bits for the exponent and 32 bits for the mantissa.
The C64 also uses another zero page area called "ARG" (Floating Point ARGument). This acts as a secondary workspace for floating-point numbers. The FAC and ARG often work together during calculations.
See Also