Chapter 18. Machine Words and Bits

The MDL facility for dealing with uninterpreted machine words and bits involves two data TYPEs: WORD and BITS. A WORD is simply an uninterpreted machine word, while a BITS is a "pointer" to a set of bits within a WORD. Operating on WORDs is usually done only when compiled programs are used (chapter 19).

18.1. WORDs

A WORD in MDL is a PDP-10 machine word of 36 bits. A WORD always PRINTs in "# format", and its contents are always printed in octal (hence preceded and followed by *). Examples:

#WORD 0                  ;"all 0s"$
#WORD *000000000000*

#WORD *2000*             ;"one bit 1"$
#WORD *000000002000*

#WORD *525252525252*     ;"every other bit 1"$
#WORD *525252525252*

WORD is its own PRIMTYPE; it is also the PRIMTYPE of FIX, FLOAT, CHARACTER, and any other TYPE which can fit its data into one machine word.

A WORD cannot be an argument to +, -, or indeed any SUBRs except for CHTYPE, GETBITS, PUTBITS and several bit-manipulating functions, all to be described below. Thus any arithmetic bit manipulation must be done by CHTYPEing a WORD to FIX, doing the arithmetic, and then CHTYPEing back to WORD. However, bit manipulation can be done without CHTYPEing the thing to be played with to a WORD, so long as it is of PRIMTYPE WORD; the result of the manipulation will be of the same TYPE as the original object or can be CHTYPEd to it.

18.2. BITS

An object of TYPE BITS is of PRIMTYPE WORD, and PRINTs just like a WORD. The internal form of a BITS is precisely that of a PDP-10 "byte pointer", which is, in fact, just what a BITS is.

For purposes of explaining what a BITS is, assume that the bits in a WORD are numbered from right to left, with the rightmost bit numbered 0 and the leftmost numbered 35, as in

35 34 33 ... 2 1 0

(This is not the "standard" ordering: the "standard" one goes from left to right.)

A BITS is most conveniently created via the SUBR BITS:

<BITS width:fix right-edge:fix>

returns a BITS which "points to" a set of bits width wide, with rightmost bit right-edge. Both arguments must be of TYPE FIX, and the second is optional, 0 by default.

Examples: the indicated application of BITS returns an object of TYPE BITS which points to the indicated set of bits in a WORD:

Example	Returns
<BITS 7>	35 ... 7 6 ... 0
<BITS 4 18>	35 ... 22 21 20 19 18 17 ... 0
<BITS 36>	35 ... 0

18.3. GETBITS

<GETBITS from:primtype-word bits>

where from is an object of PRIMTYPE WORD, returns a new object whose TYPE is WORD. This object is constructed in the following way: the set of bits in from pointed to by bits is copied into the new object, right-adjusted, that is, lined up against the right end (bit number 0) of the new object. All those bits of the new object which are not copied are set to zero. In other words, GETBITS takes bits from an arbitrary place in from and puts them at the right of a new object. The from argument to GETBITS is not affected.

Examples:

<GETBITS #WORD *777777777777* <BITS 3>>$
#WORD *000000000007*
<GETBITS *012345670123* <BITS 6 18>>$
#WORD *000000000045*

18.4. PUTBITS

<PUTBITS to:primtype-word bits from:primtype-word>

where to and from are of PRIMTYPE WORD, returns a copy of to, modified as follows: the set of bits in to which are pointed to by bits are replaced by the appropriate number of rightmost bits copied from from (optional, 0 by default). In other words: PUTBITS takes bits from the right of from and stuffs them into an arbitrary position in a copy of to. None of the arguments to PUTBITS is affected.

Examples:

<PUTBITS #WORD *777777777777* <BITS 6 3>>$
#WORD *777777777007*
<PUTBITS #WORD *666777000111* <BITS 5 15> #WORD *123*>$
#WORD *666776300111*
<PUTBITS #WORD *765432107654* <BITS 18>>$
#WORD *765432000000*

18.5. Bitwise Boolean Operations

Each of the SUBRs ANDB, ORB, XORB, and EQVB takes arguments of PRIMTYPE WORD and returns a WORD which is the bitwise Boolean "and", inclusive "or", exclusive "or", or "equivalence" (inverse of exclusive "or"), respectively, of its arguments. Each takes any number of arguments. If no argument is given, a WORD with all bits off (ORB and XORB) or on (ANDB and EQVB) is returned. If only one argument is given, it is returned unchanged but CHTYPEd to a WORD. If more than two arguments are given, the operator is applied to the first two, then applied to that result and the third, etc. Be sure not to confuse AND and OR with ANDB and ORB.

18.6. Bitwise Shifting Operations

<LSH from:primtype-word amount:fix>

returns a new WORD containing the bits in from, shifted the number of bits specified by amount (mod 256, says the hardware). Zero bits are brought in at the end being vacated; bits shifted out at the other end are lost. If amount is positive, shifting is to the left; if amount is negative, shifting is to the right. Examples:

<LSH 8 6>$
#WORD *000000001000*
<LSH 8 -6>$
#WORD *000000000000*

<ROT from:primtype-word amount:fix>

returns a new WORD containing the bits from from, rotated the number of bits specified by amount (mod 256, says the hardware). Rotation is a cyclic bitwise shift where bits shifted out at one end are put back in at the other. If amount is positive, rotation is to the left; if amount is negative, rotation is to the right. Examples:

<ROT 8 6>$
#WORD *000000001000*
<ROT 8 -6>$
#WORD *100000000000*