On 2008-12-11, Bob Campbell wrote:
"Peter Hucker" wrote in message
news
DOS simply loads TSRs, it doesn't 'manage' them.

Then what gives each program time on the CPU?

TSRs just hooked into DOS's keyboard handler, and responded when a
particular key combination was pressed, usually ALT-something.

BIOS's keyboard event interrupt, well, some did.

I wrote a few that used the 18.2Hz timer interrupt or the 1024Hz RTC
interrupt.

When this key combo was pressed DOS would hand control to the TSR.

well, no. when any key was pressed the TSR would check it and decide
if it needed to take further action.

When you
"exited" the TSR, control was passed back DOS (or whatever other app was
running at the time the ALT-something key was pressed).

DOS was merely a program loader, a disk formatter and some related
utilities. Everything else was left up to the apps - memory management,
printing, video, serial comms, etc. That is why each word processor come
with it's own printer drivers.

actually dos had rudimetary serial comms and would provide every
application with a file handle ( number 3 or 4 - I forget which) opened on
(almost always) COM1