Page 184 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes baton back to the caller. This statement can appear only once at the end of the subroutine
declaration.

13.2 Restart Sequence

The restart sequence is made up of three machine cycles
In the 1st machine cycle:

The microprocessor sends the INTA signal. While INTA is active the microprocessor reads the
data lines expecting to receive, from the interrupting device, the opcode for the specific RST
instruction.
In the 2nd and 3rd machine cycles:
The 16-bit address of the next instruction is saved on the stack.
Then the microprocessor jumps to the address associated with the specified RST instruction.

How does the external device produce the opcode for the appropriate RST instruction?
The opcode is simply a collection of bits. So, the device needs to set the bits of the data bus to the
appropriate value in response to an INTA signal.

During the interrupt acknowledge machine cycle, (the 1st machine cycle of the RST operation):
The Microprocessor activates the INTA signal. This signal will enable the Tri-state buffers, which
will place the value EFH on the data bus.

Explain, How we work with subroutines?

13.3 Conditional Call Instruction

The call instruction is used for two purposes.
1. to invoke an internal or external procedure as a subroutine

2. to activate/deactivate a condition trap subroutine handler

13.3.1 Subroutine call

call procedureName [ argumentExpression [ , argumentExpression [ , ...etc ] ]

This form of the call instruction invokes an internal or external procedure named procedureName,
or a built-in function, as a subroutine. A series of arguments can optionally be prepared by the
argument Expression values. These arguments can be acquired in the subroutine by using either
the parse arg or arg instructions, or the arg built-in function. The subroutine can optionally return
a result, which can be acquired by referencing the special RESULT variable. Often the result can
be processed directly in-place, by using a function call instead.
The name provided as procedureName can be provided as a quoted string -- e.g. 'X2C'. When it
is a quoted string, an internal procedure is not invoked, and a built-in function or external
procedure is invoked instead. When a built-in function is invoked, using this technique, the name
in quotes should be in upper case.

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