SBM

Name	Subtract DATA RAM from memory with borrow
Function	The content of the previously selected RAM character is subtracted from the accumulator with borrow.
Syntax	SBM
Assembled
Binary	11101000
Decimal	232
Hexadecimal	0xE8
Symbolic
Execution	1 word, 8-bit code and an execution time of 10.8 \({\mu}\) sec
Side-effects	Depending on the result, the carry bit is reset or set.
Implemented	sbm

Detailed Description

The value of the DATA RAM character specified by the last SRC instruction is subtracted from the accumulator with borrow. The data character is unaffected. A borrow from the previous subtraction is indicated by the carry bit being equal to one at the beginning of this instruction. No borrow from the previous subtraction is indicated by the carry bit being equal to zero at the beginning of this instruction. This instruction sets the carry bit if the result generates no borrow, and resets the carry bit if the result generates a borrow. The subtract with borrow operation is actually performed by complementing each bit of the data character and adding the resulting value plus the complement of the carry bit to the accumulator.

Notes

This instruction may be used to subtract numbers greater than 4 bits in length. The carry bit must be complemented by the program between each required subtraction operation. For an example of this, see “Decimal Subtraction”:.

The opcode for this instruction does not contain any additional data:

Example

In order to perform a normal subtraction, the carry bit should be zero.

Assume the carry bit is 1, the accumulator contains 7, and the DATA RAM character 1 of register 0 of chip 0 contains 5, the SBM will perform the following operation:

/ Example
        FIM   1P    1
        SRC   1P
        SBM

The sbm operation above is carried out as follows:

      Accumulator   =   0 1 1 1
~  Data Character   =   1 0 1 0    ( Character = 0 1 0 1)
~  Carry            =         0    ( carry = 1)
                        -------
      Result      0     0 0 0 1
              Carry indicates a borrow

The accumulator contains 1 and the carry bit is reset.