DP8344 datasheet(28/184 Pages) NSC

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20 CPU Description (Continued)

22 CPU FUNCTIONAL DESCRIPTION

221 ALU

The BCP provides a full function high speed 8-bit Arithmetic

Logic Unit (ALU) with full carry look ahead signed arithme-

tic and overflow decision capabilities The ALU can perform

six arithmetic nine logic one rotate and two shift operations

on binary data Full access is provided to all CPU registers

as both source and destination operands and using the in-

direct addressing mode results may be placed directly into

data memory All operations which have an internal destina-

tion (register addressing) are completed in two (2) T-states

External destination operations (indirect addressing to data

memory) complete in three (3) T-states

Arithmetic operations include addition with or without carry

and subtraction with or without borrow (represented by car-

ry) Subtractions are performed using 2’s complement addi-

tion to accommodate signed operands The subtrahend is

converted to its 2’s complement equivalent by the ALU and

then added to the minuend The result is left in 2’s comple-

ment form

The remaining ALU operations include full logic shift and

rotate operations The logic functions include Complement

AND

Exclusive-OR

Compare and Bit Test

Zero

through seven bit right and left shift operations are provided

along with a zero through seven bit right rotate operation

Note that the shift and rotate operations may only be per-

formed on a register which is both the source and destina-

tion (See the Instruction Set Overview section for detailed

descriptions of these operations)

The BCP ALU provides the programmer with four instruction

result status bits for conditional operations These bits

(known as condition code flags) indicate the status (or con-

dition) of the destination byte produced by certain instruc-

tions Not all instructions have an affect on every status flag

(See the Instruction Set Reference section for the specific

details on what status flags a given instruction affects)

These flags are held in the Condition Code Register

CCR

see

Figure 2-7

76543210

BIRQ

where

N e Negative

C e Carry

V e Overflow

Z e Zero

FIGURE 2-7 Condition Code Register ALU Flags

If an instruction is documented as affecting a given flag

then the flags are set (to 1) or cleared (to 0) under the

following conditions

The Negative flag is set if the most significant bit

(MSB) of the result is one (1) otherwise it is cleared

This flag represents the sign of the result if it is inter-

preted as a 2’s complement number

The Carry flag is set if

a) An addition operation generates a carry see

Fig-

ure 2-8a

b) A subtract or compare operation generates a bor-

row see

Figure 2-8b

c) The last bit shifted out during a shift operation (in

either direction) is a one (1) see

Figure 2-9

d) The last bit rotated by the rotate operation is a one

(1) see

Figure 2-10

In all other conditions C is cleared

Overflow is set whenever the result of an arithmetic or

compare operation on signed operands is not repre-

sentable by the operand size thereby producing an

incorrect result For example the addition of the two

signed negative numbers in

Figure 2-8a would set V

since the correct representation of the result both

sign and magnitude is not possible in 8 bits On the

other hand in

Figure 2-8b and 2-8c

V would be

cleared because the results are correctly represented

in both sign and magnitude It is important to remem-

ber that Overflow is only meaningful in signed arith-

metic and that it is the programmer’s responsibility to

determine if a given operation involves signed or un-

signed values

The Zero flag is set only when an operation produces

an all bits cleared result (ie a zero) In all other con-

ditions Z is cleared

11101010

10111010

11011100

10001100

11000100

01100011

w 01110110

x 11110110

w 00111111

C e 1

V e 1

V e 0

(a)

(b)

(c)

FIGURE 28 Carry and Overflow Calculations

TLF9336 – D3

FIGURE 2-9 Shifts’ Effect on Carry

TLF9336 – D4

FIGURE 2-10 Rotate’s Effect on Carry

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