Keil C

Abstract

The use of C language to program micro controllers is becoming too common. And most of the time its not easy to buld an application in assembly which instead you can make easily in C. So It's important that you know C language for microcontroller which is commonly known as Embedded C. As we are going to use Keil C51 Compiler , hence we also call it Keil C.All C code compiled and linked using the Keil tools will begin at address 0x4000 in code memory. Such code may not be programmed into devices with less than 16Kbytes of Read-Only Memory. Code written in assembly may circumvent this limitation by using the "origin" keyword to set the start to address 0x0000. No such work-around exists for C programs, though. However, the integrated debugger in the evaluation software may still be used for testing code. Once tested, the code may be compiled by the full version of the Keil software, or by another compiler that supports the C extensions used by Keil.

Introduction of Keil C

The Keil C compiler has made some modifications to an otherwise ANSI-compliant implementation of the C programming language. These modifications were made solely to facilitate the use of a higher-level language like C for writing programs on micro controllers

In writing applications for a typical computer, the operating system handles manages memory on behalf of the programs, eliminating their need to know about the memory structure of the hardware. Even more important, most computers having a unified memory space, with the code and data sharing the same RAM. This is not true with the 8051, which has separate memory spaces for code, on-chip data, and external data.

To accommodate for this when writing C code, Keil added extensions to variable declarations to specify which memory space the variable is allocated from, or points to

These extensions may be used as part of the variable type in declaration or casting by placing the extension after the type, as in the example below. If the memory type extension is not specified, the compiler will decide which memory type to use automatically, based on the memory model (SMALL, COMPACT, or LARGE, as specified in the project properties in Keil).

Using :

Since the processor only save the current program counter before executing an interrupt handler, the handler can potentially damage any data that was in the registers prior to the interrupt. This in turn would corrupt the program once the processor goes back to where it left off. To avoid this, the Keil compiler determines which registers will be used by the interrupt handler function, pushes them out to the stack, executes the handler, and then restores the registers from the stack, before returning to the interrupted code.

However, this incurs extra time, especially if a lot of registers will be used. It is preferred that as little time be spent in interrupts as possible. To decrease this time, Keil provides an optional extension, using , to the interrupt extension that tells the compiler to simple change to a new register bank prior to executing the handler, instead of pushing the registers to the stack

In the 8051, interrupts have two possible priorities: high and lo. If, during the processing of an interrupt, another interrupt of the same priority occurs, the processor will continue processing the first interrupt. The second interrupt will only be processed after the first has finished. However, if an interrupt of a higher priority arrives, the first (low priority) interrupt will itself be interrupted, and not resume until the higher priority interrupt has finished. Because of this, all interrupts of the same priority may use the same register bank.

The using extension should be used when quick execution time is of high importance, or when other functions are called from the interrupt handler, as it would otherwise push all of the registers on to the stack prior to calling the function, incurring more time penalties.