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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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