11.4 The Carry Flag as a Bit Accumulator

The btx, shift, and rotate instructions all set or clear the carry flag depending on the operation and/or selected bit. Because these instructions place their "bit result" in the carry flag, it is often convenient to think of the carry flag as a 1-bit register or accumulator for bit operations. In this section we will explore some of the operations possible with this bit result in the carry flag.

Instructions that will be useful for manipulating bit results in the carry flag are those that use the carry flag as some sort of input value. The following is a sampling of such instructions:

adc, sbb
rcl, rcr
cmc (we'll throw in clc and stc even though they don't use the carry as input)
jc, jnc
setc, setnc

The adc and sbb instructions add or subtract their operands along with the carry flag. So if you've computed some bit result into the carry flag, you can figure that result into an addition or subtraction using these instructions. This isn't a common operation, but it is available if it's useful to you.

To merge a bit result in the carry flag, you most often use the rotate through carry instructions (rcl and rcr). These instructions, of course, move the carry flag into the L.O. or H.O. bits of their destination operand. These instructions are very useful for packing a set of bit results into a byte, word, or double word value.

The cmc (complement carry) instruction lets you easily invert the result of some bit operation. You can also use the clc and stc instructions to initialize the carry flag prior to some string of bit operations involving the carry flag.

Of course, instructions that test the carry flag are going to be very popular after a calculation that leaves a bit result in the carry flag. The jc, jnc, setc, and setnc instructions are quite useful here. You can also use the HLA @c and @nc operands in a boolean expression to test the result in the carry flag.

If you have a sequence of bit calculations and you would like to test to see if the calculations produce a specific set of 1-bit results, the easiest way to do this is to clear a register or memory location and use the rcl or rcr instructions to shift each result into that location. Once the bit operations are complete, then you can compare the register or memory location against a constant value to see if you've obtained a particular result sequence. If you want to test a sequence of results involving conjunction and disjunction (i.e., strings of results involving ANDs and ORs), then you could use the setc and setnc instruction to set a register to zero or one and then use the and/or instructions to merge the results.