You can use the standard C way (note this is assuming a 32-bit register (e.g. PSoC 5 device).
volatile uint32_t * myReg = (volatile uint32_t *)0xE000E304;
uint32_t regVal = *myReg;
*myReg = 0x0000FFFF;
PSoC also provides macros and typedefs for this process.
Cypress fully describes all of the fixed PSoC 5LP registers in this PDF. Note that this does not include registers that are generated when you add components to the visual editor (e.g. UART and ADC registers).