Mnemonic

Description

Purpose of this instruction is to subtract a 32-bit operand that is in core storage from the contents of the accumulator and accumulator extension. The accumulator and accumulator extension must be loaded with the desired operand before the subtract-double operation is performed. A load-double instruction can be used to load the accumulator and accumulator extension.

The operand addressed by the subtract-double instruction must start at an even-word address in core storage. If an odd-word address is specified by the subtract-double instruction, the single word at that address is subtracted from the accumulator and from the accumulator extension.

Except for the size of the operands, the subtract-double operation proceeds in much the same manner as the subtract operation:

1. The result of the subtract-double operation is placed in the accumulator and accumulator extension.
2. Result of the operation is either positive or negative, depending upon the magnitude and signs of the participating operands. (For a list of the magnitude and size of operands that give the sign of the result, see "Subtract Instruction".)
3. The value in the accumulator and accumulator extension has a sign as signified by bit 0 in the accumulator. The leftmost bit of each operand determines that operand's sign.
4. The operand in core storage is unchanged by the operation.

Pictorially, the operation proceeds as follows:

Except for the fact that the operand addressed should start at an even word address, there are no addressing exceptions for the subtract-double instruction; all forms of addressing that are described under "Effective-Address Generation" apply to the SD instruction.

Indicators: The carry indicator is automatically reset to 0 at the beginning of execution of a subtract- double instruction. If, during execution of the subtract-double instruction, a borrow occurs to the left of the high-order (leftmost) position of the accumulator, then the carry indicator is set to 1 (on); if no such borrow occurs, the carry indicator remains reset. It can subsequently be set or reset by the various actions listed under "Carry and Overflow Indicators" (see Figure 13).

The overflow indicator must be reset to 0 if it is to be used during execution of a subtract-double instruction. If the overflow indicator is at a value of 1 at the start of a subtract-double operation, it is not changed regardless of the result of the operation.

If the overflow indicator is at a value of zero at the start of a subtract-double operation, it is set to a value of 1 if the subtraction produces a result that exceeds the capacity of the accumulator plus the accumulator extension. For example, assume that the number 1 is subtracted from the largest negative number that can be held in the accumulator and accumulator extension:

 S

 1 000 0000 0000 0000 0000 0000 0000 0000    In accumulator and extension
-0 000 0000 0000 0000 0000 0000 0000 0001    From core storage
 0 111 1111 1111 1111 1111 1111 1111 1111

(S = sign bit)

The capacity of the accumulator and accumulator extension is exceeded because subtracting 1 from a negative number should produce a negative number result that is 1 greater than the original number. But the result is clearly a positive number (bit 0 in the accumulator equals 0). Therefore, for this operation, the overflow indicator is set on.

Maximum capacity of the accumulator and accumulator extension is:

Refer to "Carry and Overflow Indicators" for a discussion of how these two indicators can be used together in certain arithmetic operations.

Examples

Subtract Double

But wait, there's MORE...