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Additional Data Types
The instruction prefix 173703, when it precedes an instruction,
causes that instruction to act on data belonging to one of a number
of additional data types other than those which are used in the
instructions seen so far.
These types are:
- Register Packed Decimal
- This is an alternative form of
packed decimal number which, like binary integers, comes in
a few fixed sizes for loading into the arithmetic/index registers of the
computer.
- Simple Floating
- This data type uses the arithmetic/index
registers of the computer for floating-point arithmetic, but with an
unusually simple floating-point format, consisting of one signed
integer containing the exponent, followed by a signed integer of
equal length containing the mantissa. This allows a very wide range
of exponents.
- Decimal Exponent Floating-Point
- This data type specifies a
floating-point number which is stored in the regular floating-point
registers, and which includes an exponent and a mantissa which
are both binary, but in which the exponent is interpreted as a power
of ten. This allows calculations which have an exact result when
decimal arithmetic of the specified precision is performed to yield
that result; historically, this data type was used in John von Neumann's
interpreter for the JOSS language.
- Compressed Decimal
- Decimal numbers, even when they are in the
packed format which takes four bits per digit, make less efficient use
of memory than binary numbers. This has been one reason people no longer
tend to consider making computers that are decimal instead of binary,
despite the fact that this would make them easier for people to understand.
But three digits can be very efficiently packed into ten binary bits.
For this reason, at IBM, a method of representing three digits in ten bits,
called Chen-Ho encoding, was devised that can be performed much more quickly
than converting three digits to a ten-bit binary integer. The compressed
decimal type is a type analogous to packed decimal which makes use of
this form of encoding.
- Register Compressed Decimal
- A type is also provided which applies
Chen-Ho encoding to register packed quantities.
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