How does the SysCtlPeripheralEnable() set the Control Register bit?

Hi, buddy,

I have a function call TIVA C project like this:

ROM_SysCtlPeripheralEnable( SYSCTL_PERIPH_TIMER2);

it is same as : 

SysCtlPeripheralEnable( SYSCTL_PERIPH_TIMER2);

where,

#define SYSCTL_PERIPH_TIMER2    0xf0000402     //Timer 2

if we want to enable TIMER2 clock, what we need to do is just set the bit 2 in RCGCTIMER register, it's address is 0x400F.E604.

but i substitute the parameter in the function

SysCtlPeripheralEnable( SYSCTL_PERIPH_TIMER2), and extend the last Macro HWREG and get:

(*(volatile uint32_t *) (0x43FC_C088)) = 1;

I am  wondering , How can this  operation set the bit2 in RCGCTIMER?

Best Regards and Thanks.