Re: Disobeying jet engines - why?

John Larkin wrote:
On Fri, 25 Jan 2008 15:38:15 +0000, Martin Brown
<|||newspam|||@nezumi.demon.co.uk> wrote:



I do embedded realtime apps that have no OS. I've written a few
RTOS's, but haven't needed to use one in a long time.

If the code is...

START: do thing1
do thing2
do thing3
goto START

where is the OS?

The OS is the bit that should step in when one of thing1, thing2 or thing3 fails to complete. Most non-trivial embedded applications have some kind of RT kernel underpinning the allocation of resources.


I doubt it. You can do arbitrarily non-trivial apps with just state
machines running in a loop. The only RTOS would then be a hardware
watchdog timer.

The code blocks can themselves use whatever resources they want,
assuming they are neither hostile nor stupid. The rare
allocation-of-resources situation can be handled by code blocks
locking out interrupts, or with simple flags.

If ever you assign a programmer to do a deep-embedded app, and he says
"first, we'll have to write/buy an rtos", fire him instantly.


John

LOL! I had a programmer write code for a user interface module. 4x20 LCD, 3 LEDs, 5 buttons and a MODBUS interface (it was a master). Used a 8951 with 64kb flash and 4kb SRAM. He wrote an RTOS, which wasnt a bad start, although not really necessary. but ran out of RAM, and used most of the FLASH, which was supposed to store drive config data - I expected the code to take 4kb max.

turns out his C compiler was using 32-bit numbers for BOOLEANS! and it got a lot worse. It was a really fancy RTOS, multi-threaded & multi-tasking. And got binned. I learned a lot about writing clear specs from that debacle. The replacement programmer got all the code in 8kb including the GNU real-time debugger. and used a fraction of the RAM.

Cheers
Terry
.