M422: Handling Packed Vectors of Bytes

Author(s): J. Zoll, C. Letertre	Library: KERNLIB
Submitter:	Submitted: 28.01.1971
Language: Fortran or Assembler	Revised: 16.09.1991

PACBYT allows handling of packed vectors of bytes. Any such vector consists of a string of bytes, all of NBITS bits, INWORD of them packed into one computer word, stored from right to left.

Structure:

SUBROUTINE and FUNCTION subprograms
User Entry Names: PKBYT, UPKBYT, JBYTPK, SBYTPK
External References: JBYT, SBYT (Fortran version)

Usage:

The 2-word vector MPACK specifies the packing parameters:

    MPACK(1) = NBITS
    MPACK(2) = INWORD

is accepted as specifying both and INWORD equal to the number of bits per word on the given computer.

    CALL PKBYT(IB,MX,JX,N,MPACK)

packs the N-word vector IB of small integers into the bytes JX,JX+1,...,JX+N-1 of the byte-vector MX.

    CALL UPKBYT(MA,JA,IY,N,MPACK)

unpacks the N bytes JA,JA+1,...,JA+N-1 of the packed byte-vector MA into the vector IY of small integers.

    IX = JBYTPK(MA,JA,MPACK)

fetches the JA-th byte from the packed byte-vector MA.

    CALL SBYTPK(IT,MX,JX,MPACK)

sets the first byte from IT into the JX'th byte of the packed byte vector MX.

Notes:

1. These routines, and the manner of packing byte-vectors, is compatible with the routines JBYT and SBYT (M421), except that there the location of a byte in the word is specified, whereas here the ordinal number of a byte in the vector has to be given. The conversion is as follows:

   The byte with ordinal number J is found in word ,
   on byte starting at bit .

2. Bits and bytes are numbered from right to left within one and the same computer word; across a word boundary there is a jump from the most significant part of the current word to the least significant part of the next word.