ST20-C1 STMICROELECTRONICS [STMicroelectronics], ST20-C1 Datasheet - Page 21

• Manufacturer Part Number: ST20-C1
• Description: Instruction Set Reference Manual
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

3.3.1
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.
3.3.2
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
register is set to the value given in Table 7.3.
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
Status register
Evaluation stack
Areg
Breg
Areg
Breg
Creg
Creg
Figure 3.5 Pushing a value x onto the evaluation stack
Figure 3.6 Popping a value from the evaluation stack
Before
Before
a
b
c
a
b
c
After
After
x
a
b
b
c
a
3 Architecture
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