AMD AMD-K6-2/400 User Guide - Page 263
System Management Mode (SMM), 12.1 SMM Operating Mode and Default Register Values - processor driver
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23542A/0-September 2000 Preliminary Information AMD-K6™-2E+ Embedded Processor Data Sheet 12 System Management Mode (SMM) 12.1 SMM is an alternate operating mode entered by way of a system management interrupt (SMI#) and handled by an interrupt service routine. SMM is designed for system control activities such as power management. These activities appear transparent to conventional operating systems like DOS and Windows. SMM is targeted for use by the Basic Input Output System (BIOS), specialized low-level device drivers, and the operating system. The code and data for SMM are stored in the SMM memory area, which is isolated from main memory. The processor enters SMM by the assertion of the SMI# interrupt and the processor's acknowledgment by the assertion of SMIACT#. At this point the processor saves its state into the SMM memory state-save area and jumps to the SMM service routine. The processor returns from SMM when it executes the resume (RSM) instruction from within the SMM service routine. Subsequently, the processor restores its state from the SMM save area, negates SMIACT#, and resumes execution with the instruction following the point where it entered SMM. The following sections summarize the SMM state-save area, entry into and exit from SMM, exceptions and interrupts in SMM, memory allocation and addressing in SMM, and the SMI# and SMIACT# signals. SMM Operating Mode and Default Register Values The software environment within SMM has the following characteristics: s Addressing and operation in real mode s 4-Gbyte segment limits s Default 16-bit operand, address, and stack sizes, although instruction prefixes can override these defaults s Control transfers that do not override the default operand size truncate the EIP to 16 bits s 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 Chapter 12 System Management Mode (SMM) 241