NonStop S-Series Server Description Manual (G06.24+)

Memory Addressing and Access

HP NonStop S-Series Server Description Manual—520331-003

4-8

Address Formats

The 32-bit address used for each of the four addressing spaces is identified by the bit

pattern of the first one, two, or three bits. See Figure 4-4. The one-bit pattern of “0”

specifies a nonprivileged space address (relative). The three-bit pattern of “100” or

“101” specifies a Kseg0 or Kseg1 address, respectively (physical). The two-bit pattern

of “11” specifies a Kseg2 address.

Any address with a 0 in the most significant bit position is a nonprivileged space

address. All the remaining bits are then available to specify a particular byte within that

space. Bits 1 through 6 specify a region, 7 through 14 specify a unitary segment, 15

through 17 a page, and 18 through 31 a byte. Any address with a 1 in the most

significant bit position is in the higher-address half of the process address space and

therefore in the privileged space.

In nonprivileged space, it is sometimes convenient to ignore the region boundaries and

treat bits 1 through 14 of the relative address as a relative segment number.

All addresses in the privileged space begin with a 1 in the most significant bit position.

If the next-most significant bit is a 0, the address is a physical address, either within the

Kseg0 space or within the Kseg1 space. If the third-most significant bit is a 0, the

address is within Kseg0; if that bit is a 1, the address is within Kseg1. In either case,

the field consisting of bits 3 through 17 specifies a frame in physical memory, and the

field consisting of bits 18 through 31 specifies the byte within that frame. (Physical

memory does not use the concept of segments, as virtual memory does.)

If the two most significant bits are both 1, the address is within the Kseg2 space. Like

the nonprivileged space, Kseg2 addresses are organized into regions, unitary

segments, and pages. Bits 2 through 6 specify a region, 7 through 14 specify a unitary

segment, 15 through 17 a page, and 18 through 31 a byte. (Unlike the nonprivileged

space, only five bits are available to specify a region, because bit 1 is used to

distinguish the Kseg2 address from Kseg0 and Kseg1 addresses; thus 32 regions are

available instead of 64.)