ST20-C1 datasheet(21/205 Pages) STMICROELECTRONICS

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3 Architecture

3.3.1

Evaluation stack

The registers Areg, Breg and Creg are organized as a three register evaluation stack,

with Areg at the top. The evaluation stack is used for expression evaluation and to

hold operands and results of instructions. Generally, instructions may pop values from

or push values onto the evaluation stack or both, and do not address individual evalu-

ation stack registers.

Pushing a value onto the stack means that the value initially in Breg is pushed into

Creg, the value in Areg is pushed into Breg and the new value is put in Areg.

Popping a value from the stack means that a value is taken from Areg, the value

initially in Breg is popped into Areg, and the value in Creg is popped into Breg.The

value left in Creg varies between instructions, but is generally the value initially in the

Areg. These actions are illustrated in Figure 3.5 and Figure 3.6.

Figure 3.5 Pushing a value x onto the evaluation stack

Figure 3.6 Popping a value from the evaluation stack

3.3.2

Status register

The status register contains status bits which describe the current state of the

executing process and any errors which may have been detected. Initially the status

The contents of the status register are summarized in Table 3.2 and described in more

detail in the following paragraphs. Generally the status register is local except for the

Before

Areg

Breg

Creg

After

Before

Areg

Breg

Creg

After

ST20-C1 Datasheet(PDF) 21 Page - STMicroelectronics