Hitech C Syntax

Question

Can someone explain me this syntax? It comes from the Hi Tech C include file

    /*  STATUS bits */
static volatile bit IRP     @ (unsigned)&STATUS*8+7;
static volatile bit RP1     @ (unsigned)&STATUS*8+6;
static volatile bit RP0     @ (unsigned)&STATUS*8+5;
static volatile bit TO  @ (unsigned)&STATUS*8+4;
static volatile bit PD  @ (unsigned)&STATUS*8+3;
static volatile bit ZERO    @ (unsigned)&STATUS*8+2;
static volatile bit DC      @ (unsigned)&STATUS*8+1;
static volatile bit CARRY   @ (unsigned)&STATUS*8+0;

Answer 1

I presume these are peripherail hardware registers. The bit type and @ are non-standard. @ places them at absolute addresses given by STATUS. bit tells the compiler that the addresses are actually single bits, so it possibly has to use appropriate instructions (bit-operations).

According to @LPs' comment (and after some thought), this looks like PIC-MCU (you did not state the CPU used). The bit type tells the compiler the addresses of the objects (ZERO, etc.) address single bits in the "RAM" (STATUS is actually a CPU register memory-mapped) address space. The bit-number is packed into the lower 3 bits (bit 0..7) and the byte-address is in the upper bits.

The right side of the @ calculats this bit-address: (8 bits/byte, hence the multiplication) and bit-number (lower 3 bits, hence the addition. Alternatively one could use bit-operators (identical result):

static volatile bit IRP @ ((unsigned)&STATUS << 3) | 7;
...

I'm very sure, @ and bit are explained in the compiler documentation.

Note that the bit-type is actually violating the C standard, as this mandates the smallest addressable type to be char with at least 8 bits and sizeof(char) == 1.