Logical AND
The AND function of two bits is given by the following truth table:
Since any bit ANDed with a zero produces a zero, and any bit ANDed with a one
remains unchanged, the AND function is often used to zero groups of bits.
The following subroutine produces the AND, bit by bit, of the two 4-bit
quantities held in index registers 0 and 1.
The result is placed in register 0, while register 1 is set to 0. Index registers 2 and 3 are also used.
For example, if register 0 = 1110 and register 1 = 0011, register 0 will be replaced with 0010.
1 1 1 0
AND 0 0 1 1
-------
0 0 1 0
1 1 1 0
AND 0 0 1 1
-------
0 0 1 0
The subroutine produces the AND of two bits by placing the bits in the leftmost position of the accumulator and register 2, respectively, and zeroing the right-most three bits of the accumulator and register 2. Register 2 is then added to the accumulator, and the resulting carry is equal to the AND of the two bits.
AND, FIM 1P 11 / Register 2 = 0, Register 3 = 11
L1, LDM 0 / Get bit of Register 0, Set accumulator = 0
XCH 0 / Set Register 0 to accumulator; Register 0 = 0
RAL / Move first 'AND' bit to carry
XCH 0 / Save shifted data in Register 0, set accumulator = 0
INC 3 / Done if Register 3 = 0
XCH 3 / Register 3 to accumulator
JCN 4 L2 / Return if accumulator = 0
XCH 3 / Otherwise, restore accumulator and Register 3
RAR / Bit of Register 0 is alone in the accumulator
XCH 2 / Save first 'AND' bit in Register 2
XCH 1 / Get bit of Register 1
RAL / Move leftmost bit to carry
XCH 1 / Save shifted data in Register 1
RAR / Move second 'AND' bit to carry
ADD 2 / 'ADD' gives the 'AND' of the bits in carry
JUN L1
L2, BBL 0 / Return to main program
