Intermec 6200 Reference Guide

R

Subfunction 1, of Interrupt 15h, is traditionally reserved for cassette interface

control.  The 4000 Series BIOS replaced it with a set of multiĆtasking services

informally referred to as INT 15.  4000API.EXE provides this same set of

services for 4000 Series multiĆtasking applications.  These services are

nonstandard, but 4000API.EXE may be executed on any PC to provide these

services.

INT 15 manipulates two linked lists of task contexts, the ready list and the

pended list.  The pended list contains tasks that are waiting for some event.  A

task moves from this list to the ready list when its timeout expires or when the

event on which it is waiting occurs.
The ready list is a queue of tasks that are scheduled for execution.  Tasks enter

at the tail of the queue and are activated from the head.  All tasks have the

same priority and are executed roundĆrobin.
The currently executing task is not on either list.  It moves to the pended list if it

issues wait any," pend on mailbox, pend on queue, or delay with timeout

commands.  The current task moves to the ready list if its time slice expires

(preemptive only) or if it voluntarily gives up control by issuing a delay with 0

timeout.
Int15 defaults to cooperative multiĆtasking.  That means that every task must

occasionally give up control of the processor to let other tasks execute.
By enabling timeĆslicing, int15 becomes a preemptive multiĆtasker.  Each task in

the ready list is allocated a 55Ćmillisecond time slice.  If the currently executing

task exhausts its time slice before relinquishing control of the processor, it is

forcibly swapped out and placed on the tail of the ready list.

Functions that require timeout values as input expect the register pair DXCX to

contain the number of milliseconds before timeout.  The timer resolution is the

same as the system timer ticks:  55 milliseconds.  A task that requests a

1 millisecond time out is guaranteed to wait at least 1 millisecond and possibly

up to 55 milliseconds.

Most of the BIOS device handling services are not reentrant.  Tasks cannot share

the same device (the display, for example) without an arbitrator that makes sure

that both tasks do not try to use the BIOS at the same time.  This resource

arbitration can be handled via the intertask communication provided by

mailboxes and queues.

A mailbox is a 4Ćbyte variable used for messageĆpassing between tasks.  A

mailbox is empty if it contains 0; otherwise, it contains a message.  A task can

perform three mailbox operations:  post, pend, and accept.
Posting is the action of placing a 4Ćbyte message into a mailbox.  Posting to a

mailbox that is not empty overlays the message that was in the mailbox.
When a task pends on an empty mailbox, the task is moved to the pended list

until another task posts a message to the mailbox.  When a task pends on a

mailbox with a message in it, the pend request returns without pending.
A task can accept from a mailbox if it wants to retrieve a message from the box

without pending.  An accept from an empty box returns 0.