\ FORTH BIT Arrays
\ by Bob Dickow ( AKA Budsy) dickow@turbonet.com
\ Feb 26, 2018
\ Notes:
\ endian agnostic, address unit agnostic
\ Vocabulary (internal dependencies) ^2MASKS bmask _BIT@
\ Vocabulary: 2^ BIT@ BIT! BOOL@ BOOL!
\ BOOL@ and BOOL! are convenience words to avoid converting FALSE to 1
\ 2^ may be handy outside the array access words.
\ Buffers for a bit array should be a minimum of 1 CELL in size
DECIMAL
CELL 8 * CONSTANT AddressUnitBits \ 8 is bits in a byte
\ calulates the power of 2 ( n -- 2^n )
\ (will yield 0 on overflow, by the way)
: 2^ ( n -- n )
2 SWAP DUP 1 > IF
1 DO 2 * LOOP ELSE
NIP DUP 2 = NOT IF 1+
THEN
THEN
;
\ compiling word, creates an array of CELL * 8 powers of 2 for masking use in the array fetch and store words.
\ does: ( nindex -- nvalue ) \\ nindex is 0, 1, 2 ... giving 2^31, 2^30, 2^29 ... 2^0
: ^2MASKS ( -- )
CREATE
0 AddressUnitBits 1- do i 2^ , -1 +loop
does> swap CELLS + @
;
^2MASKS bmask \ lookup table of powers of 2 for the BIT@ and BOOL@ words
\ factored out, internal use by BIT@, BOOL@
: _BIT@ ( caddr n -- n )
AddressUnitBits /MOD ( caddr n n )
CELLS ( caddr n n )
ROT + ( n caddr )
@ SWAP ( nval n )
bmask AND ( n )
;
\ given a memory buffer address caddr and index n representing at bit-wise
\ index, return the value of the bit at that index as 1 | 0.
: BIT@ ( caddr n -- n )
_BIT@ ( n )
0= NOT
ABS
;
\ store a value TRUE|FALSE or 1|0, given the value, an address, and a bit-wise index
\ (synonym for BOOL! below)
: BIT! ( n caddr n -- )
ROT ABS IF ['] OR ELSE ['] XOR THEN >R \ set masking op
AddressUnitBits /MOD ( caddr n n )
CELLS ( caddr n n )
ROT + DUP ( n caddr caddr )
@ ROT ( caddr caddr nval )
bmask R> EXECUTE ( caddr n )
SWAP !
;
\ given a memory buffer address caddr and index n representing at bit-wise
\ index, return the value of the bit as a TRUE | FALSE value.
: BOOL@ ( caddr n -- flg )
_BIT@ ( n )
0= NOT ( n )
;
\ Synonym for BIT!
\ store a value TRUE|FALSE ( or 1|0 ), given the value n, a base address of the array, and a bit-wise index into it.
: BOOL! ( n caddr n -- )
BIT!
;