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Central Arithmetic Logic Section

4-9

Central Processing Unit

4.3.1

Central Arithmetic Logic Unit (CALU)

The CALU implements a wide range of arithmetic and logic functions, most of

which execute in a single clock cycle. These functions can be grouped into four

categories:

- 16-bit addition

- 16-bit subtraction

- Boolean logic operations

- Bit testing, shifting, and rotating

Because the CALU can perform Boolean operations, you can perform bit ma-

nipulation. For bit shifting and rotating, the CALU uses the accumulator. The

CALU is referred to as central because there is an independent arithmetic unit,

the auxiliary register arithmetic unit (ARAU), which is described in Section 4.4.

A description of the inputs, the output, and an associated status bit of the CALU

follows.

Inputs. The CALU has two inputs (see Figure 4–6):

- One input is always provided by the 32-bit accumulator.

- The other input is provided by one of the following:

J The product-scaling shifter (see section 4.2.2)

J The input data-scaling shifter (see Section 4.1)

Output. Once the CALU performs an operation, it transfers the result to the

32-bit accumulator, which is capable of performing bit shifts of its contents. The

output of the accumulator is connected to the 32-bit output data-scaling shifter.

Through the output shifter, the accumulator’s upper and lower 16-bit words

can be individually shifted and stored to data memory.

Sign-extension mode bit. For many but not all instructions, the sign-exten-

sion mode bit (SXM), bit 10 of status register ST1, determines whether the

CALU uses sign extension during its calculations. If SXM = 0, sign extension

is suppressed. If SXM = 1, sign extension is enabled.

4.3.2

Accumulator

Once the CALU performs an operation, it transfers the result to the 32-bit accu-

mulator, which can then perform single-bit shifts or rotations on its contents.

Each of the accumulator’s upper and lower 16-bit words can be passed to the

output data-scaling shifter, where it can be shifted and then stored in data

memory. The following describes the status bits and branch instructions

associated with the accumulator.

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