Method for Distinguishing Between a 286-Type Central Processing Unit and a 386-Type Central Processing Unit

This invention relates to a method for detecting whether the CPU of a "Intel 86"-family computer system is an 80386 ("386") or an 80286 ("286"). The 386 and 286 CPUs are described in detail in publications by and available from Intel Corporation, Santa Clara, Calif., including the "Intel 80386 Hardware Reference Manual," Order No. 231732-1, the "Intel 80386 Programmer's Reference Manual," Order No. 230985-1, and the "Intel 80386 System Software Writer's Guide," Order No. 231499-1, to which reference is hereby made.

FIG. 1 is copy of commented source code of an assembly language routine for practicing the invention.

FIG. 2 is a flow chart of the assembly language routine of FIG. 1.

In accordance with the present invention, a method is disclosed for distinguishing between the 386 and 286 CPUs. The NT bit flag (i.e., bit 14) and the IOPL bits (bits 12 and 13) of the 386 flags word are always zero in the 286 architecture and cannot be set, but can be set to a one on the 386. The method includes the steps of attempting to set any of bits 12, 13, or 14 and examining the results.

A method in accordance with the invention includes the steps of (1) creating an image of the flags word of the CPU; (2) setting the NT and IOPL bits in the image; (3) up-dating the flags word with the contents of the image; (4) updating the image with the new contents of the flags word; and (5) determining whether any of the NT bit or the IOPL bits in the updated image is set. If such bit was successfully set, the CPU is a 386, otherwise a 286.

Referring now to FIGS. 1 and 2, the operation of the method according to the preferred embodiment of the invention will now be described. FIG. 1 is a listing of the code according to the preferred embodiment of the invention, using the well-known mnemonics for the assembly language instruction set of microprocessors manufactured and sold by Intel Corporation, such as the 80286 and 80386 microprocessors. FIG. 2 is a flow chart of the code of FIG. 1; the following description will be directed to the flow chart of FIG. 2.

In process 10, the contents of the flags register of the microprocessor are pushed onto the stack of the microprocessor. Process 20 sets the contents of the general purpose register AX to all zeroes. In process 30, the contents of the AX register are loaded into the flags register; as listed in FIG. 1, this is accomplished by pushing the contents of the AX register onto the stack, followed by popping the contents of the top register in the stack into the flags register. Process 40 begins the process of examining the contents of the flags register by pushing the contents of the flags register onto the stack, and popping the top register in the stack into register AX. Process 42 examines bits 12 through 15 of register AX by performing the logical AND of the contents of the AX register with the value 0F00Ch, and comparing the result to the value 0F000h. If the result of the comparison indicates that none of bits 12 through 15 of the flags register were set to zero in process 30, the microprocessor is neither an 80286 nor an 80386, but instead is an 8086 or 8088. Process 45 loads register AX with the value 086h (indicating that the microprocessor type is neither an 80286 nor an 80386); control then passes to process 50 for restoration of the flags register with its prior contents, which were stored in the stack via process 10.

If the result of process 42 indicates that all of bits 12 through 15 of register AX were zero after process 30, process 60 next loads register AX with the value 0F000h, placing ones in bits 12 through 15 thereof and zeroes in all other bit locations. Process 70 loads the flags register with the contents of the AX register by pushing the contents of register AX into the stack, and by popping the top stack register into the flags register. Process 80 next copies the contents of the flags register into register AX, by pushing the flags register contents onto the stack and popping the top stack register contents into register AX. Process 90 tests the value of the bits 12 through 15 to see if any were set to one; this is done in the code of FIG. 1 by performing the logical AND of register AX with the value 0F000h, and conditionally branching on a zero result. If none of the bits were set to one (i.e., the logical AND returned a zero result), the microprocessor is of the 80286 type because the flags register bits 12 through 15 could not be set to one; in this event, process 92 loads register AX with the result 286h, and the flags register contents are restored by process 50. If any of the bits were set to one by process 70 (i.e., the logical AND did not have a zero result), indicating that the microprocessor is of the 80386 type, process 94 sets register AX to the value 386h in process 94, and the prior contents of the flags register are restored therein in process 50.

After performing the routine of FIGS. 1 and 2, the contents of the AX register may be examined in conventional fashion, to read the result of the method of FIGS. 1 and 2 of operating the microprocessor to identify its type.

The method may also include making a determination whether the CPU is an 8088 or 8086 (also by Intel). Such a determination may be made, for example, by examining the MSB (bit 15) of the flag word in the real mode, which is always a one on the 8086 and 8088 and always a zero on the 286 and 386. The method of making the determination is not claimed as a part of the present invention.