C function access to R0 to R12 registers - greenhills

Question

I need to write the C function that will return the value of a specific hardware register. For example R0.
I am unclear from the GHS documentation how this is done with the macros provided by the GHS Compiler.
uint32_t readRegRx(void)
{
uint32_t x = 0U;
__asm("MOV Rx, ??");
return x;
}
What is the syntax in the GHS compiler for referencing a local variable as an argument to an inline assembly instruction?
I've seen this in the GHS documentation:
asm int mulword(a,b)
{
%con a %con b
mov r0,a
mov r1,b
mul r0,r1,r0
%con a %reg b
mov r0,a
mul r0,b,r0
%reg a %con b
mov r0,b
mul r0,a,r0
%reg a %reg b
mul r0,a,b
%con a %mem b
mov r0,a
ldr r1,b
mul r0,r1,r0
%mem a %con b
ldr r0,a
mov r1,b
mul r0,r1,r0
%mem a %mem b
ldr r0,a
ldr r1,b
mul r0,r1,r0
%error
}
But this isn't exactly what I want, I think. The example from the documention above describes a function taking arguments. The return value is implicitly in R0.
In my case, what I want is to use a plain C function, with embedded inline assembly to read a register (R-anything) and store the value in a local variable in the function.

Answer 1

I received information from the GHS support and it addresses the means to read hardware registers (Rn) within C functions analogous to the extended GNU ARM features. The following applies to GHS compiler usage, not GNU compiler usage:
"For asm macro purposes (use within an asm macro), it's probably best to use the enhanced GNU asm syntax, and you'll want to turn on "Accept GNU asm statements" (--gnu_asm)."
int bar(void)
{
int a;
__asm__("mov r0, %0" : : "r"(a)); // move a to r0. Replace r0 to suit.
// Stuff
return a;
}
Alternative method:
asm void readR0(r0Val)
{
%reg r0Val
mov r0Val,r0
}
void foo(void)
{
register int regValReg = 0;
readR0(regValReg);
// Stuff
}