IT2354 Embedded System - Discuss data operations in ARM Process

1.      Discuss data operations in ARM Process.

ANS:        

Ø  Data Operations

Arithmetic and logical operations in C are performed in variables. Variables are implemented as memory locations. Therefore, to be able to write instructions to perform C expressions and assignments, we must consider both arithmetic and logical instructions as well as instructions for reading and writing memory.

Let us consider sample fragment of C code with data declarations and several assignment statements. The variables a, b, c, x, y, and z all become data locations in memory. In most cases data are kept relatively separate from instructions in the program’s memory image.

In the ARM processor, arithmetic and logical operations cannot be performed directly on memory locations. While some processors allow such operations to directly reference main memory, ARM is load-store architecture. Data operands must first be loaded into the CPU and then stored back to main memory to save the results. Figure 2.8 shows the registers in the basic ARM programming model. ARM has 16 general-purpose registers, r0 through r15. Except for r15, they are identical. Any operation that can be done on one of them can be done on the other one also. The r15 register has the same capabilities as the other registers, but it is also used as the program counter. The program counter should of course not be overwritten for use in data operations. However, giving the PC the properties of a general-purpose register allows the program counter value to be used as an operand in computations, which can make certain programming tasks easier.

The other important basic register in the programming model is the current program status register (CPSR). This register is set automatically during every arithmetic, logical, or shifting operation. The top four bits of the CPSR hold the following useful information about the results of that arithmetic/logical operation:

ü  The negative (N) bit is set when the result is negative in two’s-complement arithmetic.

ü  The zero (Z) bit is set when every bit of the result is zero.

ü  The carry (C) bit is set when there is a carry out of the operation.

ü  The overflow (V) bit is set when an arithmetic operation results in an overflow.