6-24 System Design
AMD-K5 Processor Technical Reference Manual 18524C/0—Nov1996
6.3.1 Operating Mode and Default Register Values
The software environment in SMM has the following features:
■ Addressing as in Real mode
■ 4-Gbyte segment limit
■ Default 16-bit operand, address, and stack size, although
instruction prefixes can override these defaults
■ Control transfers that do not override the default operand
size truncate the EIP to 16 bits
■ Far jumps or calls cannot transfer control to a segment with
a base address requiring more than 20 bits, as in Real mode
segment-base addressing.
■ A20M is not recognized (unlike the Pentium processor)
■ Interrupt vectors use the Real-mode interrupt vector table
(but see Section 6.3.8 on page 6-32)
■ The IF flag in EFLAGS is cleared (INTR not recognized)
■ The NMI interrupt is disabled
■ The TF flag in EFLAGS is cleared (single-step traces dis-
abled)
■ Debug register DR7 is cleared (debug traps disabled)
Figure 6-2 on page 6-7 shows the default map of the SMM mem-
ory area. It consists of a 64-Kbyte area, between 0003_0000h
and 0003_FFFFh, of which the top 32-Kbytes (0003_8000h and
0003_FFFFh) must be populated with RAM. The default code-
segment (CS) base address for the area—called the SMM Base
Address—is at 0003_0000h. The top 512 bytes (0003_FFFFh to
0003_FE00h) contain a fill-down SMM state-save area. The
default entry point for the SMM service routine is at
0003_8000h.
Table 6-1 shows the initial state of registers when entering
SMM.
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