The Pentium processor is equipped with a super-debugging mode, called probe mode. When in probe mode, the Pentium processor ceases to execute any x86 instructions, and all bus activity stops. Probe mode is used by In-Circuit-Emulators to interface with the Pentium, much as ICE-mode was used by previous generations of Intel x86 processors. Probe mode is superior to previous generations of Intel x86 processors which had a dedicated ICE-mode. ICE-mode on previous x86 processors was really System Management Mode (but Intel never told you that). By implication, ICE-mode was executing code on behalf of the ICE host computer. This is one of the drawback to ICE mode which makes probe mode superior. In probe mode, no x86 instructions are executing, and the only bus activity which is performed, is at the direction of the probe mode ICE.
Probe mode implements a control register called the Probe Mode Control Register (PMCR). The PMCR is instrumental for enabling the undocumented ICEBP instruction (see the ICEBP instruction description for details on how to use this instruction to your advantage). Other bits in this register enable and disable the performance monitoring/breakpoint pins. The only other known bit in this register is a read-only bit indicating that the microprocessor has entered System Management Mode (SMM).
Back to Pentium Model-Specific Registers article by Ralf Brown.