In addition to faster components, two significant design improvements were incorporated: base registers and additional hardware instructions. The two 18-bit base registers (one for instruction storage and one for data storage) permitted dynamic relocation: as a program got swapped in and out of main memory, its instructions and data could be placed anywhere each time it got reloaded. To support multiprogramming, the 1108 had memory protection using two base and limit registers, with 512-word resolution. One was called the I-bank or instruction bank, and the other the D-bank or data bank. If the I-bank and D-bank of a program were put into different physical banks of memory, a 1/2 microsecond advantage accrued, called "alternate bank timing." The 1108 also introduced the Processor State Register, or PSR. In addition to controlling the Base Registers, it included various control "bits" that enabled the various Storage Protection features, allowed selection of either the User or Exec set of A, X & R registers, and enabled "Guard Mode" for user programs. Guard Mode prevented user programs from execution of Executive Only "privileged" instructions, and from accessing memory locations outside the program's allocated memory.