Intermec 6200 Reference Guide

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This interrupt is intercepted as a means of monitoring when the system is busy.

A lookup table determines which system function represents activity.  For each of

the functions whose AH values are marked in that table, NPOWER performs a

read from a system busy port" prior to passing that function on to DOS for

execution.  The default value used as the system busy port" is 3ffh; the scratch

pad register for COM1.  This default can be changed automatically by the APM

BIOS, simply by supporting the APM OEM BIOS extension Int 15h, AX=5380h,

BX=C200h.  This is a relatively new addition to the portable APM OEM BIOS

specification, so it is expected that it is not supported at first, thus the need for a

default port value to use when the function call is not supported.
Function 40h (Write to File or Device) is handled somewhat differently.  With

that function, BX is also checked to see which file handle is used.  Only those file

handles greater than or equal to three denote system activity.  In other words,

writing to stdout is not recorded as activity.
The lookup table is used by INT 28h to determine when idling is performed.  For

functions between 00h and 0Ch, the lookup value is saved each time a call is

intercepted.  See INT 28h for how that value is used.

This interrupt is issued by DOS from the standard input/output functions

whenever it believes that it is idle.  Unfortunately, that includes situations such

as the time spent polling on a COM port when data is being sent to or received

from a COM port, as in the case where the console is redirected via TTY.  Thus, a

write string call directed to a COM port would run very slowly if idling occurred

every time INT 28h is invoked.  If NPOWER is disconnected, this interrupt is not

intercepted.
To prevent a slow down, INT 28h looks at the value saved by INT 21h to

determine which function was last performed.  Only those DOS functions that

perform input from the console are allowed to cause idling out of INT 28h.  By

modifying that saved value under some circumstances, INT 28h idles if:

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The last recorded DOS call is an input function and at least two INT 28h

calls have been issued since that time.  The second call is first to idle.

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If the last recorded DOS call was function 0Ch (Flush Buffer and Read

Standard Input), idling occurs with the first INT 28h call received.

Note only those DOS calls in the range of 00h through 0Ch are even recorded.

These DOS function calls represent the functions to be recorded as activity:

AH

AL Function description  

11

Find first matching file using FCB

12

Find next matching file using FCB

13

Delete file using FCB

14

Sequential read from FCB file

15

Sequential write to FCB file

16

Create or truncate file using FCB

17

Rename file using FCB

21

Read random record from FCB file

22

Write random record to FCB file

27

Random block read from FCB file