Commodore 128 Programmer's Reference Manual page 162

152 COMMODORE 128
U
This program segment essentially performs the following mathematical operation: U>
(10 + 4)-6+10=18.
The first instruction clears the carry bit. The second instruction loads the accumu- (
lator with $0A (10 decimal). The third instruction stores the value in address $FB for J
later use. The fourth instruction adds the constant $04 to the value already in the
accumulator. The SBC instruction subtracts the constant $06 from the contents of the ) .
accumulator. The next instruction, ADC $FB, adds the contents of memory location [
$FB to the contents of the accumulator. The resulting value (18($12)) of all the ^
mathematical operations is stored in address $FD.
)
LOGICAL INSTRUCTIONS u
(AND, EOR, AND ORA)
These instructions operate on the contents of a memory address and a register. The AND > ,
operation is a binary (Boolean) algebra operation having two operands that can result in | j
one of two values, 0 or 1. The only way an AND operation can result in a 1 is if both
the operands equal 1; otherwise the result is 0. For example, the two operands are the
contents of a specified memory address and the contents of the accumulator. Here's an ) j
illustration of this concept: U
Memory address = 10001010 | /
Accumulator = 11110010 jj
Result of AND = 10000010
As noted, the result of an AND operation is (true) 1, only if the two operands are L)
equal to 1; otherwise the result is 0. Notice bit 7 (high-order bit) equals 1 because both
bit 7's in the operands are 1. The only other resulting bit equal to 1 is bit 1, since both bit ^ i
l's are equal to 1. The rest of the bits are equal to zero since no other bit positions in jj
both operands are equal to 1. A 1 and a 0 equals 0, as does a 0 and a 0.
The Boolean OR works differently. The general rule is: , f
If one of the operands equals 1, the resulting Boolean value equals 1. LJ
For example, the two operands are the contents of a specified memory address and
the contents of the accumulator. Each individual bit can be treated as an operand. Here's j
an illustration. !—'
Contents of Memory Address = 10101001 j
Contents of Accumulator = 10000011 L.
Result of the OR operation = 10101011
For all the bit positions that equal one in either operand, the resulting value of that ^
bit position equals 1. The result is 1 if either operand or both operands are equal to 1.
The exclusive OR works similarly to the OR operation, except if both operands I J
equal 1, the result is zero. This suggests the following general rule: (j