3.5. Use of the OpenRISC 1000 l.nop Instruction

3.5. Use of the OpenRISC 1000 `l.nop` Instruction

The OpenRISC 1000 no-operation instruction, l.nop (32'h1500_0000), can take an optional 16-bit immediate parameter, which forms the least significant 16-bits of the instruction word. This value is ignored by the CPU, but may be monitored by test benches

In ORPSoC this is used to provide I/O and control functions for the C code running on the processor.

l.nop 1 (32'h1500_0001). Terminates execution, with the value in GPR 3 as return code. Thus the C library routine exit is implemented as:
```
void exit (int i)
{
  asm("l.add r3,r0,%0": : "r" (i));
  asm("l.nop %0": :"K" (NOP_EXIT));
  while (1);
}
	  
```
l.nop 2 (32'h1500_0002). Provides a reporting function. The value in GPR 3 is printed in hex.
l.nop 3 (32'h1500_0003). Provides printf functionality, with the arguments passed according to the OpenRISC 1000 Application Binary Interface (ABI). Not currently implemented.
l.nop 4 (32'h1500_0004). An Embecosm addition. The least significant byte of GPR 3 is printed as a character. Thus the C function putc can be implemented as:
```
void putc(int value)
{
  asm("l.addi\tr3,%0,0": :"r" (value));
  asm("l.nop %0": :"K" (NOP_PUTC));
}
	  
```
More complex library routines (to print strings, numbers etc) can then be built up from this.