multitasking in ARM7

The above code which I posted, is a [low quality] demonstration for:
A simple supervisory loop processing each module would almost certainly be sufficient.

void Timer0Handler (void) __irq
{
  timer_counter++;

  if ( timer_KEYpress != 0 )
    timer_KEYpress--;
  else
    KEYS_check();

  if ( timer_GPIO8LED != 0 )
    timer_GPIO8LED--;

  if ( timer_MSPI7LED != 0 )
    timer_MSPI7LED--;

  T0IR = 1;                     /* clear interrupt flag */
  VICVectAddr = 0;              /* Acknowledge Interrupt */
}

void GPIO8LED_show(void)
{
        static BYTE GPIO8LED_on = 1;
        static BYTE GPIO8LED_i = 0;

        if ( ( timer_GPIO8LED == 0 ) && ( GPIO8LED_show_on ) )
        {
                if (GPIO8LED_on)
                {
                        IOSET1 = ~( LED8SET & ( 7 << (18+GPIO8LED_i) ) );
                }
                else
                {
                        IOCLR1 = LED8SET & ( 7 << (18+GPIO8LED_i) );
                        if ( GPIO8LED_i == 7 )
                                { GPIO8LED_i = 0; }
                        else
                                { GPIO8LED_i++; }
                }
                GPIO8LED_on = 1 - GPIO8LED_on;
                timer_GPIO8LED = 8;
        }
}


void MSPI_init(void)
{
        //PINSEL0 = (PINSEL0 & 0xFFFF00FF) | 0x00005500;    // Connect SPI
        PINSEL0 = (PINSEL0 & (~(0xFF << 8))) | (0x55 << 8) ;
        IODIR0  = HC595_CS;
    S0SPCCR = 0x52;                         // Set SPI Clock
        S0SPCR  = (0 << 3) |                      // CPHA = 0, [del]
                          (1 << 4) |                      // CPOL = 1, SCK [del]
                          (1 << 5) |                      // MSTR = 1, SPI [del]
                          (0 << 6) |                      // LSBF = 0, SPI [del]
                          (0 << 7);                               // SPIE = 0, [del]
}


void MSPI7LED_show(void)
{
        static BYTE MSPI7LED_i = 0;
        BYTE const HEX_TAB[16] = {
        //     0     1     2     3     4     5     6     7     8     9
                0xC0, 0xF9, 0xA4, 0xB0, 0x99, 0x92, 0x82, 0xF8, 0x80, 0x90,
        //         A     b     C     d     E     F
                0x88, 0x83, 0xC6, 0xA1, 0x86, 0x8E };

        if ( ( timer_MSPI7LED == 0 ) && ( MSPI7LED_show_on ) )
        {
                IOCLR0 = HC595_CS;                              // 74HC595
                SPI_SPDR = HEX_TAB[MSPI7LED_i];
                while( 0 == (SPI_SPSR & 0x80) );            // Wait
                IOSET0 = HC595_CS;
                if ( MSPI7LED_i == 15 )
                        { MSPI7LED_i = 0; }
                else
                        { MSPI7LED_i++; }
                timer_MSPI7LED = 95;
        }
}

I wrote this at 2008.Nov.04 =>

int main (void)
{
        char LFnCR[] = "\n\r";

    init_VIC();
    UARTInit(9600);     /* baud rate setting */
        init_timer();

        enable_timer(0);

        PINSEL2 = PINSEL2 & (~0x08);    // for GPIO 8LED
        IODIR1  = LED8SET;              // for GPIO 8LED

        MSPI_init();

    while (1)
    {                           /* Loop forever */

                GPIO8LED_show();

                MSPI7LED_show();

                UART0rbREAD();

                if (cmd_ready)
                {
                        UART0sbWRITE( (BYTE *)cmd_string, strlen((const char *)cmd_string) );
                        UART0sbWRITE( (BYTE *)LFnCR     , 2 );
                        UART0sendKICK();
                        memset( cmd_string, 0x00, 32 );
                        cmdstr_i = 0;
                        cmd_ready = 0;
                }

                KEYS_control();

    }
        // return 0;
}


void KEYS_check(void)
{
        IOPIN0_last = IOPIN0_this;
        IOPIN0_this = IOPIN0;

        IOPIN0_stat = IOPIN0_last ^ IOPIN0_this & IOPIN0_last;

        if ( ( IOPIN0_stat & KEY1 ) == KEY1 )
                KEY1_pressed = 1;

        if ( ( IOPIN0_stat & KEY2 ) == KEY2 )
                KEY2_pressed = 1;

        if ( ( IOPIN0_stat & KEY3 ) == KEY3 )
                KEY3_pressed = 1;

        if ( ( IOPIN0_stat & KEY4 ) == KEY4 )
                KEY4_pressed = 1;

        if ( ( IOPIN0_stat & KEY5 ) == KEY5 )
                KEY5_pressed = 1;

        if ( ( IOPIN0_stat & KEY6 ) == KEY6 )
                KEY6_pressed = 1;

        timer_KEYpress = 2;
}


void KEYS_control(void)
{
        static BYTE GPIO8LED_pos = 1;
        static BYTE MSPI7LED_pos = 0;
        BYTE const HEX_TAB[16] = {
        //     0     1     2     3     4     5     6     7     8     9
                0xC0, 0xF9, 0xA4, 0xB0, 0x99, 0x92, 0x82, 0xF8, 0x80, 0x90,
        //         A     b     C     d     E     F
                0x88, 0x83, 0xC6, 0xA1, 0x86, 0x8E };

        if ( KEY1_pressed == 1 )
        {
                GPIO8LED_show_on = 1 - GPIO8LED_show_on;
                KEY1_pressed = 0;
        }

        if ( KEY4_pressed == 1 )
        {
                MSPI7LED_show_on = 1 - MSPI7LED_show_on;
                KEY4_pressed = 0;
        }

        if ( GPIO8LED_show_on == 0 )
        {
                if ( KEY2_pressed == 1 )
                {
                        if ( GPIO8LED_pos > 1 )
                                GPIO8LED_pos--;
                        KEY2_pressed = 0;
                }
                if ( KEY3_pressed == 1 )
                {
                        if ( GPIO8LED_pos < 8 )
                                GPIO8LED_pos++;
                        KEY3_pressed = 0;
                }
                IOCLR1 =    1 << (17+GPIO8LED_pos)  ;
                IOSET1 = ~( 1 << (17+GPIO8LED_pos) );
        }

        if ( MSPI7LED_show_on == 0 )
        {
                if ( KEY5_pressed == 1 )
                {
                        if ( MSPI7LED_pos > 0 )
                                MSPI7LED_pos--;
                        KEY5_pressed = 0;
                }
                if ( KEY6_pressed == 1 )
                {
                        if ( MSPI7LED_pos < 15 )
                                MSPI7LED_pos++;
                        KEY6_pressed = 0;
                }
                IOCLR0 = HC595_CS;
                SPI_SPDR = HEX_TAB[MSPI7LED_pos];
                while( 0 == (SPI_SPSR & 0x80) );
                IOSET0 = HC595_CS;
        }
}

As Andy has already mentioned, multitasking is not necessary for what you've described.

A simple supervisory loop processing each module would almost certainly be sufficient.

Search keil forum for "RTX". Also inspect your "Options for target...", Target page ("operating system").

do I need to install some operating system like real time Linux???

I am working with LPC2148. In this project, I am displaying data on LCDs which is continuously changing. also I want to read input ports.

can it be done like operating system working??? one program for checking i/p ports and second to display data on LCDs. only one is executing for fixed time and after timeout storing its status on stack and second is executing for fixed time..

Almost every microcontroller can do it.

Of course it's possible - even an 8051 can do it!

But it certainly isn't necessary just for what you've described!

is it possible?????

Yes, it is.