Pointer

Pointer-less C Compiler For PIC18Fxxxx MCUs - PLCC18

by Mengjin Su

This web page introduces a 'new' solution to embedded programming world, a C language that doesn't have pointer and pointer related grammars and rules. Instead, it uses an explicit way to accomplish all those indirect memory(RAM and ROM) data accesses. That means, a programmer who knows C language will pick it up and get used to it quickly. In theory, if no pointer related operation needed, it works the same as the standard C, and all your knowledge and experience on C can be carried over. Moreover, in cases when MCU has small amount RAM memory (register file size, so called by Microchip), it might be more efficient to access memory indirectly than that using pointers in standard C.

Why Pointer-less C

Frankly, 'pointer', as well as its related operations, is one of the most important features in C. It brings a tremendous efficiency and power to the language. But on the other side, C programmers, especially those beginners, sometimes complain about the ambiguity and the confusing using pointer in their programming. Today, a lot of MCUs used in embedded applications are in Harvard architecture that allocates ROM and RAM into two separate spaces. Each one needs to be accessed using different scheme or instructions (e.g. all the MCU products from Microchip are in this case). So the standard C pointer and its grammar notations may or may not work.

Pointer-less, Under the hood

PLCC uses different rules or syntax to do the same thing as that in standard C. In PLCC, only those basic data types exist: char, int and long (currently, 'float' data type has not been implemented in PLCC18). The following tables illustrate how PLCC works to access the memory:

basic operations:

Standard 'C'	Pointer-less 'C'
/* declare a char's pointer / char p;	/* declare an integer that will hold an address */ int p;
*p = 10;	(p, char) = 10;
*p++ = 10;	(p++, char) = 10;
/* how to access the data in ROM? / if ( ((const char *)p) > 10 ) p++;	/* specify the operation explicitly */ if ( (p, const char) > 10 ) p++;

'struct' and 'union' operations:

Standard 'C'	Pointer-less 'C'
/* define a struct */ struct my_data { char a, b, int x, y; }; struct my_data DATA;	/* define a struct */ struct my_data { char a, b, int x, y; }; struct my_data DATA;
struct my_data *p = &DATA; p->a = 10;	int p = &DATA; (p, struct my_data).a = 10;

About PLCC18

PLCC18 is a small, efficient and easy to use compiler that supports PIC18Fxxxx series MCUs from Microchip. It works on both Windows and Linux platforms, under terminal/console mode. For convenience, we still use '.c' as extension for source files.

At present, only char(8-bit) int(16-bit) and long(32-bit) basic data types are supported (float might be supported in future). All basic arithmetic operations ('+', '-', '*', '/', '%') and logical operations ('&', '|', '^') are implemented. Plus, in-line assembly code insertion is allowed. And the following pre-processors are accepted:

#if expression

#else

#endif

#ifdef symbol

#ifndef symbol

#pragma

#asm

#endasm

#define symbol expression

#define symbol(arg1, arg2, ...) expression

No function recursive calling allowed in version PLCC18 v0.4x! (will be supported in higher version for those PIC devices with extended instruction set)

[1] Install the compiler

Unzip and extract all file, and then put them into a folder (e.g. PLCC). And then put the path into the environment, like

C:>\set path=%path%;C:\PLCC

[NOTE, you can make batch file (filename with extension '.bat') to set the path].

In this folder, you should see following files:

plcc18.exe - PLCC compiler

p18asm.exe - PIC18 assembler/linker

crt0.s - library routines

pic18.h - PIC18Fxxxx header file

examples(folder) - a folder that contains some example codes

[2] Compiling command:

plcc18 <options> filename1.c filename2.c ...

After compiling, if no error detected, assembly code(file) will be generated for each source file, ended with '.s'. The assembly code is very readable, and you can check what result exactly is.

where the options are:

-L generate assembly code in LIB mode

-s short jump instruction needed (default = long jump instruction)

-Dsymbol<=xxxx> define symbol (with or without value)

[3] Assembling and linking

p18asm <options> filename1.s filename2.s ...

where the options are:

-device_name(mandatory) specify PIC device (for example: -pic18f452)

-code=xxxx code start address (default = 0x0000), no space allowed

-data=xxxx RAM data start address (default = 0x0002)

-sp=xxxx specify stack pointer initial value.

[4] Resources used by Compiler

PLCC18 will use following registers for compilation:

WREG PRODL PRODH FSR0L FSR0H FSR1L FSR1H FSR2L FSR2H TABLAT TBLPTRL TBLPTRH TBLPTRU STATUS BSR

Users should pay great attention when accessing any of those register in the code.

Further more, RAM location 0x0000 and 0x0001 (first two locations in bank0) are reserved for holding function return value (lower 16 bits). User should never use these location for any purpose. Index registers are used by compiler for index addressing. Among them FSR0L and FSR0H are use as software stack pointer. FSR1L, FSR1H, FSR2L and FSR2H are used for temporary storage dynamically*. BSR is cleared at power-on reset, and should never be modified afterwards.

(NOTE*: PICC18 version 0.5x or higher will use FSR2L and FSR2H as the software stack pointer)

[5] Download PLCC18

Click here to get PLCC18 v0.43. You can download and distribute it freely. Suggestions and comments are very welcome and can be sent to '[email protected]'!