
CHAPTER 4 MEMORY MANAGEMENT SYSTEM
60
The User segment starts at address 0 and the current active user process resides in either useg (in 32-
bit mode) or xuseg (in 64-bit mode). The TLB identically maps all references to useg/xuseg from all
modes, and controls cache accessibility.
The processor operates in User mode when the Status register contains the following bit-values:
"
KSU = 10
"
EXL = 0
"
ERL = 0
In conjunction with these bits, the UX bit in the Status register selects 32- or 64-bit User mode
addressing as follows:
"
When UX = 0, 32-bit useg space is selected.
"
When UX = 1, 64-bit xuseg space is selected.
Table 4-1. Comparison of useg and xuseg
Address bit
value
Status register bit value
Segment
name
Address range
Size
KSU
EXL
ERL
UX
32-bit
A[31] = 0
10
0
0
0
useg
0x0000 0000
to
0x7FFF FFFF
2 Gbytes
(2
31
bytes)
64-bit
A[63..40] =
0
10
0
0
1
xuseg
0x0000 0000 0000 0000
to
0x0000 00FF FFFF FFFF
1 Tbyte
(2
40
bytes)
(a) useg (32-bit mode)
In User mode, when UX = 0 in the Status register, User mode addressing is compatible with the 32-bit
addressing model shown in Figure 4-5, and a 2-Gbyte user address space is available, labelled useg.
All valid User mode virtual addresses have their most-significant bit cleared to 0; any attempt to
reference an address with the most-significant bit set while in User mode causes an Address Error
exception.
In 32-bit User mode addressing, the TLB Mismatch exception vector is used for TLB misses.
The system maps all references to useg through the TLB, and bit settings within the TLB entry for the
page determine the cacheability of a reference.
(b) xuseg (64-bit mode)
In User mode, when UX =1 in the Status register, User mode addressing is extended to the 64-bit
addressing model shown in Figure 4-5. In 64-bit User mode, the processor provides a single address
space of 2
All valid User mode virtual addresses have bits 63:40 equal to 0; an attempt to reference an address
with bits 63:40 equal to 1 causes an Address Error exception.
The XTLB Mismatch exception vector is used for TLB misses.
40
bytes, labelled xuseg.