More of my philosophy about Intel 8051 controller and about preemptive and non-preemptive timesharing and more..

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Hello,



More of my philosophy about Intel 8051 controller and about preemptive
and non-preemptive timesharing and more..

I have just quickly read the following interesting paper and it says
that judicious use of cooperative tasking techniques can also often meet
an embedded system's multitasking requirements, while giving better
performance and a simpler software environment than a preemptive
multitasker, so read it carefully here:

https://users.ece.cmu.edu/~koopman/pubs/koopman90_HeavyweightTasking.pdf

And notice that it also says in the above paper that so that to meet
the requirements with cooperative multitasking you have to move the
time-critical code to interrupt-service routines. And let us look
for example at the Intel 8051 controller here:

https://www.electronicwings.com/8051/introduction-to-8051-controller

So as you notice that it has many hardware interrupts that you can
use so that to make the cooperative tasking efficient, and i think it
also comes with two clock timers interrupts that you can use to
implement preemptive multitasking if you want, and you have also to know
about interrupt latency when programming embedded systems with hardware
controllers, and you have to know that the hardware interrupts have to
get serviced fast enough and often enough, so you shouldn't disable
interrupts for too long a period of time, and just to give you an idea
, look for example at the nonbuffered communication UART (Universal
Asynchronous Receiver Transmitter) operating at 38,400 bits per second
will interrupt every 208 microseconds. This is 1/38,400*8 because they
will interrupt for every byte (8 bits), and a processor or controller
running at 25MHz executes most of its instructions in
2 or 3 system-clock periods. That would be an average of 120 nanoseconds
(1/25,000,000*3). In theory, this means you could execute as
many as 1,730 instructions in the interrupt interval. So that was only
in theory, now you have to do the reality check. You must take into
consideration that there are more interrupts than just that
communication channel. The timer interrupt will be firing off every so
often. And the communication interrupt itself will have interrupts
disabled for good period of time, and not only that, but there is also
the tasks switch that can be expensive, so you have to think about
it efficiently.

So i invite you to read my below thoughts preemptive and non-preemptive
timesharing and more so that to understand much more efficiently:

More of my philosophy about preemptive and non-preemptive timesharing
and more..

I have just took a smart look at Modula-2 language(Modula-2 is a
structured, procedural programming language developed between 1977 and
1985 by Niklaus Wirth at ETH Zurich, and he has also developed Pascal
language, read about Niklaus Wirth here:
https://en.wikipedia.org/wiki/Niklaus_Wirth), and i think Modula-2
language was among the first languages that has provided preemptive and
non-preemptive timesharing with coroutines, but the preemptive
timesharing in Modula-2 uses Interrupt handling using IOTRANSFER, but it
is best reserved for programs that will run without operating system
support. Installing an interrupt handler on a multiuser system is not
feasi­ble because doing so would affect other users. (For this reason,
IOTRANSFER is not a mandatory feature of Modula-2.) Even on single-user
systems, IOTRANSFER can be difficult to use because installing an
interrupt handler causes the old interrupt handler (which most likely
belongs to the operating system) to be lost. So this is why i think that
the best way in modern operating systems is to use non-preemptive
timesharing with coroutines, so this is why i am providing you with my
sophisticated implementation of stackful coroutines, read about it in my
thoughts below:

More of my philosophy about timesharing that is a Solution to Computer
Bottlenecks..

I invite you to look at the following very interesting video about
timesharing that is a Solution to Computer Bottlenecks:

https://www.youtube.com/watch?v=Q07PhW5sCEk

I think i am smart, and you have to understand one important thing
and it is: What is the difference between a software architect and
a software engineer?, i think there is an important difference and it
is also like abstracted in the following question:

"How it is made?"

So i think that software engineering works at a higher level than
a software architect, this is why you will notice that i am
quickly implementing a sophisticated stackful coroutines
Library and i am quickly implementing setjmp() and longjmp() with
x64 assembler or code machine, read my below thoughts about them, but
you have to know that my sophisticated stackful coroutines Library
does a kind of timesharing as in the above video, but i think that
there is two kinds of timesharing: the preemptive one, and the
non-preemptive one, but the difference is that the preemptive one does
interrupt with a timer the coroutines from an external scheduler in
a form of function, but notice below that i am implementing the
non-preemptive timesharing in my sophisticated coroutines Library, but
you have to be smart and notice that my way of doing is like the
software architect way, since i am implementing it from the lowest level
with x64 assembler routines that are part of the non-preemptive
scheduler, but not only that, but you have also to look at how i am also
implementing a
sophisticated and much more rich interface in my stackful coroutines
Library, so it is like both software achitecting and software
engineering, so here is all my below thoughts that shows how i am
implementing it quickly, so read it carefully since you have also to
know what's the problem with the stack frames when architecturing and
using the setjmp() and longjmp() so that to implement coroutines:

More of my philosophy and precision about the link of the article and more..

And notice that the link below of the article that shows the problem
of implementing coroutines with just setjmp() and longjmp()
is from the last semester of the second year of the course
called "CS4411 Operating Systems" from Michigan Technological
University, but i think i am smart and those courses are easy
for me, so i invite you to read about this course that requires
both the course of "CS3331 Concurrent Computing" and "CS3421 Computer
Organization", and here it is:

http://www.csl.mtu.edu/cs4411.ck/www/Home.html

More of my philosophy about coroutines and about setjmp() and longjmp()..

I think i am smart, and i will say that with setjmp() and longjmp()
you can implement a generator or the like, but you can not implement
coroutines with just setjmp() and longjmp(), and so that to understand
it, i invite you to read the following article that shows how when you
yield from a first function with a longjmp() to the main body of a
program and when you call another functions with longjmp(), it can make
the stack frames not work correctly, and when you understand it you will
not use setjmp() and longjmp() alone so that to implement coroutines, so
read the following article so that to understand the problem with
the stack frames, and i am understanding it easily:

https://www.csl.mtu.edu/cs4411.ck/www/NOTES/non-local-goto/coroutine.html

So this is why i have also implemented my sophisticated stackful
coroutines library so that to solve this problem, and here is my
sophisticated coroutines library and read about it and download it from
here:

https://sites.google.com/site/scalable68/object-oriented-stackful-coroutines-library-for-delphi-and-freepascal

More of my philosophy about setjmp() and longjmp() and generators and
coroutines..

I have just quickly implemented setjmp() and longjmp() in x64 assembler,
and after that i have just implemented quickly a good example of a
generator with my setjmp() and longjmp(), look at it below, and in
computer science, a generator is a routine that can be used to control
the iteration behaviour of a loop. All generators are also iterators. A
generator is very similar to a function that returns an array, in that a
generator has parameters, can be called, and generates a sequence of
values. However, instead of building an array containing all the values
and returning them all at once, a generator yields the values one at a
time, which requires less memory and allows the caller to get started
processing the first few values immediately. In short, a generator looks
like a function but behaves like an iterator. So here is my
implementations in freepascal and delphi and they are working perfectly:

Here is my first unit that implements longjmp() and setjmp() and notice
how i am saving the non-volatile registers and how i am coding it in
x64 assembler:

======


{ Volatile registers: The calling program assumes registers
RAX, RCX, RDX, and R8 through R11 are volatile.
The contents of registers RBX, RSI, RDI, RBP, RSP, and
R12 through R15 are considered non-volatile. Functions return
values in RAX. }



unit JmpLib64;

{$IFDEF FPC}
{$ASMMODE intel}
{$ENDIF}
interface

type
jmp_buf = record
RBX,
RSI,
RDI,
RSP,
RBP,
RIP,
R12,
R13,
R14,
R15: UInt64;
end;

{ setjmp captures the complete task state which can later be used to
perform a non-local goto using longjmp. setjmp returns 0 when it is
initially called, and a non-zero value when it is returning from a call
to longjmp. setjmp must be called before longjmp. }

function setjmp(out jmpb: jmp_buf): UInt64;

{ longjmp restores the task state captured by setjmp (and passed in
jmpb). It then returns in such a way that setjmp appears to have
returned with the value retval. setjmp must be called before longjmp. }

procedure longjmp(const jmpb: jmp_buf; retval: UInt64);
implementation

function setjmp(out jmpb: jmp_buf): UInt64; assembler;{$IFDEF FPC}
nostackframe; {$ENDIF}register;
asm
{ -> RCX jmpb }
{ <- RAX Result }
MOV RDX, [RSP] // Fetch return address (RIP)
// Save task state
MOV [RCX+jmp_buf.&RBX], RBX
MOV [RCX+jmp_buf.&RSI], RSI
MOV [RCX+jmp_buf.&RDI], RDI
MOV [RCX+jmp_buf.&RSP], RSP
MOV [RCX+jmp_buf.&RBP], RBP
MOV [RCX+jmp_buf.&RIP], RDX
MOV [RCX+jmp_buf.&R12], R12
MOV [RCX+jmp_buf.&R13], R13
MOV [RCX+jmp_buf.&R14], R14
MOV [RCX+jmp_buf.&R15], R15


SUB RAX, RAX
@@1:
end;

procedure longjmp(const jmpb: jmp_buf; retval: UInt64);assembler;{$IFDEF
FPC} nostackframe; {$ENDIF}register;
asm
{ -> RCX jmpb }
{ RDX retval }
{ <- RAX Result }
XCHG RDX, RCX
MOV RAX,RCX
MOV RCX, [RDX+jmp_buf.&RIP]
// Restore task state
MOV RBX, [RDX+jmp_buf.&RBX]
MOV RSI, [RDX+jmp_buf.&RSI]
MOV RDI, [RDX+jmp_buf.&RDI]
MOV RSP, [RDX+jmp_buf.&RSP]
MOV RBP, [RDX+jmp_buf.&RBP]
MOV R12, [RDX+jmp_buf.&R12]
MOV R13, [RDX+jmp_buf.&R13]
MOV R14, [RDX+jmp_buf.&R14]
MOV R15, [RDX+jmp_buf.&R15]
MOV [RSP], RCX // Restore return address (RIP)

TEST RAX, RAX // Ensure retval is <> 0
JNZ @@1
MOV RAX, 1
@@1:
end;

end.

================

And here is my example of a generator with my longjmp() and setjmp():


{ In computer science, a generator is a routine that can be used to
control the iteration behaviour of a loop. All generators are also
iterators. A generator is very similar to a function that returns an
array, in that a generator has parameters, can be called, and generates
a sequence of values. However, instead of building an array containing
all the values and returning them all at once, a generator yields the
values one at a time, which requires less memory and allows the caller
to get started processing the first few values immediately. In short, a
generator looks like a function but behaves like an iterator. }

program test_generator;

{$APPTYPE CONSOLE}

uses
JmpLib64;

type PtrInt = ^Integer;

var
childtask,maintask: jmp_buf;
myarr1: array of integer;
i,a:integer;
Ptr1:PtrInt;

function generator(var myarr:array of integer):integer;

var i1:integer;
val:integer;
ptr:PtrInt;
begin


i1:=0;

val:= setjmp(childtask);

i1:=val-1;

if val=0 then
begin
new(ptr);
ptr^:=myarr1[i1];
longjmp(maintask,uint64(ptr));
end;

if val=10
then
begin
writeln('Exiting child..');
exit;
end;

inc(i1);
new(ptr);
ptr^:=myarr1[i1];
longjmp(maintask,uint64(ptr));
end;

begin

setlength(myarr1,10);

for i:=0 to 9
do myarr1[i]:=i;

uint64(ptr1):=setjmp(maintask);

if ptr1=nil then generator(myarr1);

a:=ptr1^;
dispose(ptr1);

if (a<=length(myarr1))
then
begin
if a=length(myarr1)
then longjmp(childtask,a+1)
else
begin
writeln('Value returned by generator is: ',a);
longjmp(childtask,a+1);
end;
end;

setlength(myarr1,0);

end.

====


Thank you,
Amine Moulay Ramdane.