Microsoft Binary Format

In computing, Microsoft Binary Format (MBF) was a format for floating point numbers used in Microsoft's BASIC language products including MBASIC, QuickBasic and GW-BASIC.

MBF numbers consist of a sign bit, an eight bit exponent with bias 128, and a 23 or 55 bit significand. The decimal point is located before the assumed bit. The MBF double precision format provided slightly more precison than the IEEE 754 format, but less scale.

MBF single-precision format (32 bits):

Exponent	Sign	Significand
8 bits, bit 31-24	1 bit, bit 23	23 bits, bit 22-0
xxxxxxxx	s	mmmmmmmmmmmmmmmmmmmmmmm

MBF double-precision format (64 bits):

Exponent	Sign	Significand
8 bits, bit 63-56	1 bit, bit 55	55 bits, bit 54-0
xxxxxxxx	s	mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

By the time Visual Basic was released, the IEEE 754 standard had become widely adopted - for example, it was incorporated into Intel's 387 coprocessor and every x86 processor from the 486 on. Visual Basic uses the IEEE 754 format instead of MBF.

See also

• Floating point
• IEEE 754 — Standard for Binary Floating-Point Arithmetic
• IBM Floating Point Architecture

Further reading

• Microsoft provides a 16 bit dynamic link library with C source code containing functions to convert MBF data to IEEE 754.
• MBF to IEEE function written in Python
• MBF to IEEE conversion functions in C/C++ borrowed from old Microsoft supported code. (Pay to view)