CSC233 Assembly Language - Packed Decimal

Packed Decimal

Representation

Input data is stored in EBCDIC, or zoned decimal. Before it can be used in computations, it must be converted to a numeric format. One of the numeric formats is packed decimal.

An EBCDIC, or zoned decimal, field is packed as follows:

The first 4 bits of each byte are called the zone, and the second 4 bits of each byte are called the digit.
The zone and digit portions of the low order byte are exchanged and placed in the low order byte of the packed field.
The rest of the zones are ignored. The remaining digits are moved to the packed field, with two digits placed in each byte. If the number of digits in the zoned decimal field is even, then a zero is placed in the high order zone of the packed field.
The data is not checked to make sure it is numeric; any field can be packed. If the original field was not numeric the result may not be valid as an arithmetic operand.

Examples

Character	EBCDIC Representation	Packed Decimal Representation
1234	F1 F2 F3 F4	01 23 4F
47	F4 F7	04 7F
572150	F5 F7 F2 F1 F5 F0	05 72 15 0F
1A2B3C	F1 C1 F2 C2 F3 C3	01 12 23 3C
abcd	81 82 83 84	01 23 48
1234	40 40 F1 F2 F3 F4	00 01 23 4F
1234	F1 F2 F3 F4 40 40	01 23 40 04

Packed Fields

Use data type P with DS and DC instructions to define packed fields. A packed field contains C or F in the sign position if the number is positive, or it contains D in the sign position if the number is negative. If the field is larger than necessary to hold the given value, then it is padded on the left with zeros. If the field is not large enough to hold the value given, then the high order digits are lost.

Instruction	Storage Defined
`A DS PL4`	4 byte field for packed data
`B DC P'24680'`	24 68 0C
`C DC P'-7543'`	07 54 3D
`D DC PL5'15672'`	00 00 15 67 2C
`E DC PL3'3150083'`	50 08 3C

The PACK Instruction

The PACK instruction is an SS instruction that packs the value in the second operand and stores it in the first operand.

The second operand holds the value to be packed.
The first operand receives the packed value.
PACK can be implicit or explicit. If it is implicit, the length of each operand is used. If it is explicit, the given length is used. A length can be specified for either or both operands. The maximum length for either operand is 16.
If the receiving field is larger than necessary, the high order bits are padded with zeros.
If the receiving field is smaller than necessary, the high order digits of the zoned decimal field are truncated.
No checking is performed on the values. If the field being packed is not numeric, no error will occur in the PACK operation. However, if the field is later used in an arithmetic operation, a data exception may occur, which will cause the program to crash.

Given the following definitions:

ONE      DC     C'ABCDE'
TWO      DC     C'12345'
THREE    DC     C'297'
FOUR     DC     C'  311'
FIVE     DC     C'110  '
SIX      DC     CL8'80723745'
SEVEN    DC     CL11'12345678901'
EIGHT    DS     PL3
NINE     DS     PL6

The following table shows the results of PACK instructions. Each instruction starts with the initial data given above.

PACK Instruction	Operand 1	Operand 2
`PACK EIGHT,TWO`	12 34 5F	F1 F2 F3 F4 F5
`PACK EIGHT,THREE`	00 29 7F	F2 F9 F7
`PACK EIGHT,ONE`	12 34 5C	C1 C2 C3 C4 C5
`PACK EIGHT,SIX`	23 74 5F	F8 F0 F7 F2 F3 F7 F4 F5
`PACK EIGHT,FOUR`	00 31 1F	40 40 F3 F1 F1
`PACK EIGHT,FIVE`	11 00 04	F1 F1 F0 40 40
`PACK NINE,SIX`	00 08 07 23 74 5F	F8 F0 F7 F2 F3 F7 F4 F5
`PACK NINE,SEVEN`	12 34 56 78 90 1F	F1 F2 F3 F4 F5 F6 F7 F8 F9 F0 F1
`PACK NINE(4),TWO`	00 12 34 5F xx xx	F1 F2 F3 F4 F5
`PACK EIGHT(3),TWO(5)`	12 34 5F	F1 F2 F3 F4 F5
`PACK EIGHT(2),TWO(3)`	12 3F xx	F1 F2 F3 F4 F5
`PACK EIGHT,TWO(4)`	01 23 4F	F1 F2 F3 F4 F5