Decimal Presentation : different purpose between z

问题:

I'm learning Computer Science course and when I read to these definition, I understand. But I don't know what different purpose of two presentations and why. Here some short explanation of purpose that my book said:

Zone decimal : hightly compatible with text data.
Packed decimal : faster computing speed.

Something I want to know is:

1) in zone decimal presentation there is a zone section that duplicate every digit. Why ? I see this is no purpose :(

2) why they say zone decimal is compatible with text data and why packed decimaal is faster.

Thanks :)

回答1:

Firstly - where are you learning CS? Those terms are from the 1960s, the more common name is BCD (Binary Coded Decimal)

Zone decimal uses an entire byte for each digit. This means you can just print a number as if it was text (each 'character' stores a digit 0-9) but since there are only 10 digits and a byte can hold 256 different values this is a bit wasteful.

Packed decimal uses the fact that 4bits can store 16different values. So you can store two digits in a byte (top 4bits and bottom 4bits). This is still a bit wasteful since you only use half the capacity. But it's pretty easy to extract the two digits with just shift and mask operations.

Pretty much the only place you would see BCD these days is in some low level hardware where you want to read/x-mit a digit without using a microprocessor at all. It's easy to make a BCD counter just in transistors

but if you want to do any maths you either have to do long multiplication on each digit like you would on paper - or convert into regular ints and back again

回答2:

Both of these representations have fallen out of favor, perhaps because they are not directly supported by C, and hence all of the systems descended from Unix.

Packed decimal has an advantage in two respects: since takes up less space it can get off the bus and into the processor faster, and many CISC instruction sets have dedicated instructions for arithmetic. To quote from http://en.wikipedia.org/wiki/Packed_decimal#Packed_BCD:

Packed BCD [binary coded decimal] is supported in the COBOL programming language as the "COMPUTATIONAL-3" (an IBM extension adopted by many other compiler vendors) or "PACKED-DECIMAL" (part of the 1985 COBOL standard) data type. Besides the IBM System/360 and later compatible mainframes, packed BCD was implemented in the native instruction set of the original VAX processors from Digital Equipment Corporation and was the native format for the Burroughs Corporation Medium Systems line of mainframes (descended from the 1950s Electrodata 200 series).

Zoned decimal (http://en.wikipedia.org/wiki/Zoned_decimal#Zoned_decimal) has an easy mapping between characters on punch cards and their representation in memory, which perhaps explains your textbook's claim that it is "highly compatible with text data." As the Wikipedia article suggests, it's a term more used in IBM mainframe circles. On minis, we tended to just call it plain old decimal, PIC 9 data.

回答3:

"Zoned Decimal" in its natural environment is meant to be compatable with the EBCDIC char set .

ASCII represents numbers as x'3x' -- x'39' which display as character "0" to "9".

The EBCDIC character sets (which has its origins in Hollerith pucnched cards) uses a similar but different scheme where x'F0' is displayed as characer "0' and x'F9' is displayed as character '9'.

Punched cards had a fixed length of 80 characters in many cases 10 or 12 of these characters were eaten up with record type identifiers and sequence numbers (desperately important if you dropped a bunch of cards on the floor!). So space was at a premium. Rather than enter a "+" or "-" character next to each number an "overpunch" extra holes near the top bit of the card was used to represent a positive or negative numbers, so saving a byte.

These overpunched characters were encdoded in EBCDIC as x"D0' to x'D9" for -0 to -9 and x'C0' to x'C9' for +0 to +9 usually in the last digit of the number.

Hence the "Zoned Decimal" format. The first four bits of each byte are the Zone, the second four bits the "number" to -42 was encoded as x'F4D2'. This is more of a convention than anything else as the computer could not do anything with this format. So it needed to be encoded into "packed" format before any calculations took place. This is pretty easy s 'X'F4D2' -> x'042D' is mostly a case a grabbing the last zone then extracting the "numeric" four bits from each byte, which, could then be converted to binary.

When IBM mainframes were designed the largest group of users were banks, insurance companies and utility companies. The bulk of there processing followed this pattern.

• read punch card.
• read tape record.
• add monthly payment to balance
• store new balance on tape
• print new balance

Most of the calculations involved currency amounts and most of the results were displayed immediately. It became clear that if the machine could do the arithmetic directly on the packed decimal values you could avoid several expensive "convert to binary" and "convert to decimal" instructions. As a bonus it made it easy to place the decimal point at the correct position and perform any decimal rounding. So a great deal of work went into implementing native packed decimal instructions (zero, add, subtract, multiply, divide, shift and round etc.).

This has been the preferred currency format for IBM mainframes ever since.

For many years developers on other platforms poured scorn on the mainframers for using such an archaic format, and, only recently began to realize how difficult it was to do fixed point decimal arithmetic to the standards accountants and tax collectors expect. Thanks to the efforts of Mike_Cowlishaw and others the rest of the world has caught up with the venerable IBM 360 and Java programmers can now calculate sales tax correctly using the BigDecimal library which is based on a variation on the old packed decimal format.