UART interrupt problem

I am using Olimex's SAM7-P64 board, development tool set from YAGARTO 
OPENOCD and Insight for my debugging. I sn suing USART1 and setup for 
UART receive interrupt. The code starts and keeps looping in the main() 
routine. When I enter a character (using Hyperterminal in XP PC) the 
code stops at a debug break in the UART ISR. The ISR is coded as naked 
function. When in ISR the LR shows a value of 0xC0. As a result when 
returns from interrupt it does not go back to the main() routine. In the 
beginning of ISR I've ISR_ENTRY() and at the end ISR_EXIT() macros.

#define ISR_ENTRY() asm volatile(" sub   lr, lr,#4\n" \
                                 " stmfd sp!,{r0-r12,lr}\n" \
                                 " mrs   r1, spsr\n" \
                                 " stmfd sp!,{r1}")

#define ISR_EXIT()  asm volatile(" ldmia sp!,{r1}\n" \
                                 " msr   spsr_c,r1\n" \
                                 " ldmia sp!,{r0-r12,pc}^")

Also when in the ISR the processor mode is 0x13 which is supervisor mode 
I thought I would see 0x12 the IRQ mode. Also at the beginning of the 
ISR when SPSR is stored in R1, the SPSR[5:0] has value 0x33. The code is 
compiled in ARM mode only.

Could anyone please help me in solving this problem.

Regards.