Subtract Mnemonic

 S Description

The subtract instruction is used to subtract one 16-bit operand from the 16-bit operand in the accumulator. The accumulator must first be loaded with the 16-bit value; then the subtract instruction addresses the other operand during execution of the subtract instruction. The operand in core storage and the operand in the accumulator can have the same or different signs; negative numbers are in two's complement form.

Result of the subtraction is in the accumulator at the end of the operation. The operand addressed in core storage by the subtract instruction is unchanged by the operation. Several examples are:

Example 1

``` 0 000 0000 0000 0011 = Operand in accumulator
-0 000 0000 0000 0010 = Operand addressed by subtract instruction
0 000 0000 0000 0001 = Result placed in accumulator
```

(In decimal: 3-2=1)

Example 2

``` 1 000 0000 0000 0011 = Operand in accumulator
-0 000 0000 0000 0010 = Operand addressed by subtract instruction
1 000 0000 0000 0001 = Result placed in accumulator
```

(In decimal: -32,765 - 2 = -32,767)

Example 3

``` 1 000 0000 0000 0011 = Operand in accumulator
-1 000 0000 0000 0000 = Operand addressed by subtract instruction
0 000 0000 0000 0011 = Result placed in accumulator
```

(In decimal: -32,765 - (-)32,768 = -32,765 + 32,768 = +3)

The sign of the result is dependent upon the signs and magnitudes of both operands. Possible combinations (where operand B is always numerically greater than operand A, regardless of signs) are:

Operand in Accumulator   Operand in Core Storage   Sign of Result in Accumulator
+B - (+)A = +
+B - (-)A = +
-B - (+)A = -
-B - (-)A = -
+A - (+)B = -
+A - (-)B = +
-A - (+)B = -
-A - (-)B = +

There are no addressing exceptions for the subtract instruction; all forms of addressing that are described under "Effective-Address Generation" apply to the S instruction.

Indicators: The carry indicator is automatically reset to 0 at the beginning of execution of a subtract instruction. If, during execution of the subtract instruction, a borrow occurs beyond the leftmost position of the accumulator, the carry indicator is set to 1 (on). 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 instruction. If the overflow indicator is at a value of 1 at the start of a subtract operation, it is not changed regardless of the result of the subtract operation.

If the overflow indicator is at a value of 0 at the start of an add operation, it is set to a value of 1 if the subtraction produces a result that exceeds the capacity of the accumulator. For example, assume that the following subtraction operation is performed:

``` S

1 000 0000 0000 0000    Operand in accumulator -- a negative number
-0 000 0000 0000 0001    Operand in main storage -- a positive number
0 111 1111 1111 1111    Result in accumulator -- a positive number

(S = sign bit)
```

In the first place, subtracting any positive number from any negative number should produce a negative result. But the sign bit is at a value of 0 in the result, and an overflow has occurred. The correct answer is obtained by appending (via programming) a 1 to the left of the sign bit. The overflow indicator is turned on (set to 1) for this operation. Refer to "Carry and Overflow Indicators" for a discussion of how these two indicators can be used together in certain arithmetic operations.

Maximum capacity of the accumulator is:

Power-of-2 Notation Decimal Notation Hexadecimal Notation
+215-1 +32,767 +7FFF
-215 -32,768 -8000

Examples

Subtract

Assembler Language Coding Hexadecimal Value Description of Instruction
Label   Operation   F T
 21 25

 27 30
32 33   35..40..
S         DISP 90XX Subtract contents of CSL at EA (I+DISP) from A
S     1   DISP 91XX Subtract contents of CSL at EA (XR1+DISP) from A
S     2   DISP 92XX Subtract contents of CSL at EA (XR2+DISP) from A
S     3   DISP 93XX Subtract contents of CSL at EA (XR3+DISP) from A
S   L     ADDR 9400XXXX Subtract contents of CSL at EA (Addr) from A
S   L 1   ADDR 9500XXXX Subtract contents of CSL at EA (Addr+XR1) from A
S   L 2   ADDR 9600XXXX Subtract contents of CSL at EA (Addr+XR2) from A
S   L 3   ADDR 9700XXXX Subtract contents of CSL at EA (Addr+XR3) from A
S   I     ADDR 9480XXXX Subtract contents of CSL at EA (V in CSL at Addr) from A
S   I 1   ADDR 9580XXXX Subtract contents of CSL at EA (V in CSL at "Addr+XR1") from A
S   I 2   ADDR 9680XXXX Subtract contents of CSL at EA (V in CSL at "Addr+XR2") from A
S   I 3   ADDR 9780XXXX Subtract contents of CSL at EA (V in CSL at "Addr+XR3") from A

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