RealTime

Real time is where computation has to be done by deadline

Hard Realtime vs. Soft Realtime

ex: windows or linux or bsd or solaris

      some kind of real time guarantee (not strong)

Hard realtime - embedded devices

EX walking robot adjust balance

   System on Chip

NVRAM  = power persists despite power outage. solid state drive 

Real Addressing mode

Logical address IS Real address

Relocation register

   just adds offset to real address

Lynx OS

priority inheritance and inversion

no on demand paging

Preemptive vs. Reentrant

all kernels are reentrant, but not all are preemptive

Preemptive will stop for something mid-task,

Reentrant just makes a note of something

for real time, should be preemptive

  preemptive lends itself to bugs like race conditions though.

priority based scheduling

n * ( 2 ^ (1/n) -1 )

where n is number of processes

Earliest deadline first

(….rate monotonic scheduling … ??)

Microkernel - one small kernel that does limited amount

basically just passes caller's request to function

work for realtime systems

ex: tinyOS