programming, um, fundamentals .. ?

andzerger · 02-18-2004, 07:22 PM

take the C language:

time(*long) returns the amount of seconds that have passed since some date, its declared in time.h .. why? what are the porpuse of header files? they contain practically no code; never any conditional operations, just alot of definitions, structure declarations and some function prototypes often strangely preceded by a few __s, so wheres the actual library? all i want is a boolean clock function that goes 0 .. 1 .. 0 .. 1 like the actul hardware does

yeah okay this line of inquiry probably doesnt make a whole lot of sense, im just trying to get off the high level programming trip before i forget whats really going on in a computer..

jtshaw · 02-18-2004, 07:31 PM

The purpose of the header files is so you can see what functions there are, and what they need as arguments, without having to worry about the implementation getting in the way. I mean, you can assume the glibc functions work, so who cares how the code does it. If you really want to know, download the glibc source.

As far as timers go in hardware... they usually increment a register, and throw an interrupt when it overflows. If all it did was oscillate between 0 and 1 you would have no idea if you missed a cycle or not so it wouldn't be too useful for a timer....

andzerger · 02-18-2004, 07:58 PM

does the register clear once the interrupt is thrown?

infamous41md · 02-18-2004, 09:24 PM

all things of that nature are entirely implementation dependent. if you want to play with low level stuff learn assembly it's awesome. for example, lets say you want to get a `clock' tick in assembly language. if you're on int x86 pentiums, you can use the `rdtsc' instruction. this reads the contents of the timestamp counter register on the processor. this register is incremented once every clock cycle of the processor. the low 32 bits in eax, and the high 32 go in edx. using the low bits a way to measure small time intervals. on a 500mhz comp it overflows once every 8 seconds. if you want, you can also read the whole 64 bits and that doesnt overflow for a loooong time. if you wanted to do it in C, you could do this:

Code:

#define rdtsc(low,high) \
     __asm__ __volatile__("rdtsc" : "=a" (low), "=d" (high))

#define rdtscl(low) \
     __asm__ __volatile__("rdtsc" : "=a" (low) : : "edx")

and then pass it an unsigned integer arg to read the counter. btw, that is from /usr/src/linux/include/asm-i386/msr.h"

jtshaw · 02-18-2004, 09:51 PM

The timer interrupts on overflow, which would mean it would be cleared to 0 if it represents an unsigned number.

SciYro · 02-18-2004, 09:54 PM

the headers are just definitions so the compiler knows what to do with it, the actual libs are usualy in /lib or other places like /usr/lib, these are alrdy been compiled to from there the loader loads the required imformation into memory when the program runs

andzerger · 02-19-2004, 12:51 AM

ahh beautiful, thank you all. i got a lot of thinking to do

id like to learn assembly, thats what my inquirys all about. i just havent made it to school yet (summer) and am easily confused by some of the terms im finding in the assembly book ive got .. registers for example, now at least i have a better idea about what one is (some piece of hardware that can store a certain amount of electricity, apparantly)

anyway, thanks, now i understand the first few paragraphs of chapter one

-andrew
(software engineering student to become electronics engineering student)

bdp · 02-19-2004, 01:54 AM

check out the 'what is a shift register?' paragraphs/drawing, hope this ties software and electrical engineering ideas together a bit more...

http://www.doctronics.co.uk/4015.htm