Arithmetic and logical instructions were mostly performed against values in memory as opposed to internal registers.

The Pegasus basic instruction cycle time for add/subtract/move and logical instructions was 128 microseconds.

The processor maintains internal flag bits (a status register) which indicates the results of arithmetic and logical instructions.

The INC, DEC, and logical instructions do not.

The logical instructions consist of those for performing bitwise logical operations and conditional moves on the integer registers.

The most important one is the 'a' register or accumulator, so-called because the results of most arithmetic or logical instructions are accumulated in the 'a' register.

Masking is such a common operation that many computers provide at least one arithmetic or logical instruction which can specify a mask as well as the primary operands.

With all arithmetic and logical instructions using absolute (direct) addressing, bits 14 and 13 of the address field are used to indicate the indexing mode as follows:

Sometimes a logical instruction is provided which allows specification (in a sub-operation field) of any of these 16 operations, to be performed on the two operands of the instruction.

Along with a carry flag, a sign flag and an overflow flag, the zero flag is used to check the result of an arithmetic operation, including bitwise logical instructions.

It will be seen that binary arithmetic and logical instructions need no explicit operands, and consist only of an operation code field; such an instruction therefore has a zero - address format.

One logical instruction unique to the Model 20 was "Halt and Proceed" (HPR, operation code X'99') which would quiesce input/output and stop the processor displaying a code to the operator.

The bitwise logical instructions perform AND, NAND, NOR, OR, XNOR, and XOR between two registers or a register and literal.

The eSi-RISC instruction set includes arithmetic and logical instructions (including barrel-shift, multiply and divide), comparisons, load and stores, branches and calls as well as system level instructions to control interrupts and enter lower power states.

Logical instructions were Move Characters, Move with offset, Move Immediate, Move Numerics, Move Zones, Compare Logical Immediate, Compare Logical Characters, Edit, And immediate, Or immediate, Test under Mask, and Translate.