Remote-processing CAMBASIC Instrukcja Użytkownika Pobierz pdf (Strona 46)

Commands - 13

CALL

Statement

SYNTAX: CALL address [,m1] [,m2]. . .

PURPOSE: To execute an assembly language program at the specified address and, optionally, to pass data.

REMARK S: The specified address may be an expression and is the absolute address of the assembly language

program.

The optional data may be expressions. After conversion to 16– bit integers, the data is pushed on

the CPU stack for use by the CALLed routine.

The data could be the memory address (obtained by the VAR function) of either numeric or string

variables, if the routine is to access floating point data.

The amount of data that can be saved on the stack is limited only by available memory. When the

assembly language r outine is entered, the information passed and created is structured as follows:

SP – >

m n

m n–1

m 1

return address

HL – > stack location of return address

BC – > number of data

The data may then be popped off the stack in reverse order. Note the HL register pair point to the

location of the return address in the stack, allowing a clean return to CAMBASIC. The BC register

pair contains the number of data passed to the routine.

Data m ay be returned to C AM BASIC from the assembly language routine by storing them in

memory locations before returning to CAM BASIC, and then reading the memory locations from

within CAMBASIC with the PEE K function. Or they may be stored directly in CAMBASIC

variables, if the VAR function was used to pass the variable addresses to the assembly language

program.

NOTE1: Once you have called a n assembly language r outine, you are in complete control.

CAM BASIC has no influence (unless the inter rupts ar e left enabled). If you do not re turn to

CAMBASIC, you probably have not restored the CPU stack or registers, destroyed RAM reserved

for CAM BASIC, or have not executed a RET (C9H). However, if interrupts have been enabled by

CAM BASIC, you must consider the consequences or turn off the interrupts.

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