Whether in real or protected mode, the CPU stores the base address of each segment in hidden registers called descriptor cache registers. Each time a segment register is loaded, the segment base address, segment size limit, and access attributes (access rights) are loaded, or "cached," into these hidden registers. To enhance performance, all subsequent memory references are made via the descriptor cache registers instead of calculating the physical address, or looking up the base address in the descriptor table. Understanding the role of these hidden registers is paramount for exploiting highly advanced programming techniques, and for exploiting the undocumented instruction LOADALL. Figure 2a shows the descriptor cache layout for the 80286, and figure 2b shows the layout for the 80386, and 80486.

At power-up, the descriptor cache registers are loaded with fixed, default values; the CPU is in real-mode, and all segments are marked as read/write data segments, including the Code Segment (CS). According to Intel, each time any segment register is loaded in real-mode, the base address is 16x the segment value, while the access rights and size limit attributes are given fixed, "real-mode compatible" values. This is not true. In fact, only the CS descriptor cache gets loaded with fixed values each time the segment register is loaded. Loading any other segment register in real mode doesn't change the access rights or the segment size limit attributes stored in the descriptor cache registers. For these segments, the access rights and segment size limit attributes are "honored" from any previous setting (see figure 3). Thus it is possible to have a 4G-byte Read-Only data segment in real-mode on the 80386, but Intel won't acknowledge, or support this mode of operation.

Protected mode differs from real mode in this respect: as each time a segment register is loaded, the descriptor cache register gets fully loaded; no values are "honored." The descriptor cache is loaded directly from the descriptor table. The CPU checks the validity of the segment by testing the access rights in the descriptor table. Complete checks are made, and illegal values will generate exceptions. Any attempt to load CS with a read/write data segment will generate a protection error. Likewise, any attempt to load a data segment register as an executable segment will also generate an exception. The CPU enforces these "protection" rules very strictly. If the descriptor table entry passes all the tests, then the descriptor cache register gets loaded.


Anomalies exist in the real mode handling of the access-rights attributes. Using LOADALL, it is possible to prove that these anomalies exist by directly loading the segment descriptor cache registers (access rights), then performing a series of tests that demonstrate the CPU behavior. The tests provided in DESCRIPT.ASM and its accompanying include files demonstrate that the CPU honors the access rights in the descriptor cache registers, and with the exception of CS, never changes those access attributes.Furthermore, by setting access attributes for data segments that would be illegal in protected mode (setting the executable bit), it is possible to observe undocumented CPU behavior.

Each test subgroup performs the same tests as each other. Separate subgroups are defined because each subgroup has a unique aspect that requires special attention. For example, making SS read-only requires being prepared to recover from a CPU RESET. Therefore, the subgroups are defined as follows: 1) DS, ES, FS, and GS; 2) SS; 3) CS.

DS, ES, FS, and GS require no special handling. The DS (etc.) test is broken down into two sub-subgroups DS, FS, and ES, GS to allow access to the data segment during tests where segment access is prohibited.

SS and CS require separate, special handling. SS must be handled separately to accommodate a stack that can be marked as inaccessible (read-only), or not present. Such cases will force the CPU to RESET, as any exception will generate a double-fault, which in turn will triple-fault the CPU and RESET it. CS and SS require further special handling to accommodate expand-down segments. SS expand down is simple, but CS requires moving the entire code segment into the expand-down area, along with any exception handlers -- and be able to detect if the expand down segment has been cleared by the CPU. The CS test is further differentiated from other tests in the way it must handle the expand-down CS segment. Not only does the entire code segment need to be relocated, as do exception handlers, but the test introduces the concept of CS_BIAS (called EXPANSION_BIAS in the code). CS BIAS is 0 in all cases except when the expansion direction bit is being tested. CS BIAS is used to BIAS all destination address, including those affected by IRET. When the expansion direction bit is being tested, CS BIAS is set to the two's-compliment code segment size (-CS_SIZE) -- as this is where the code segment will start IF expand-down is recognized in real mode (as it is).

I have found that in real mode, the only segment load that causes the access rights to be changed is a FAR JuMP -- which reloads the CS access rights to real-mode compatible values (93h). The segment limit is still honored, as is the CS32 (USE32) default operand size attribute.



I have written source code to demonstrate that the CPU does not behave consistently with respect to real mode vs. protected mode. Intel documentation states that the descriptor register cache's get fixed default values in real mode. The source code demonstrates that the descriptor cache registers retain -- or honor -- whatever values they have received from protected mode, or by using LOADALL.

The source code consists of many modules, and a batch file to compile the multiple files. The two main files: DESCRIPT.ASM and DESC32.ASM must be linked together. The batch file A.BAT will assemble and link these modules together. Because the Microsoft LINK utility will not allow one to link 16-bit and 32-bit segments together (whose segment names are the same), I have provided a utility that I have written to change the segment definition bit in .OBJ files to USE16 or USE32. This will allow you to LINK these modules together. Just type CVTUBIT with no arguments to get a list of the syntax.

View source code:

Download entire source code archive:

Download CVTUBIT utility:

Descritpor Table Access Rights

The definition of these bits is exactly as that of the access rights in the descriptor table, with the following exceptions:

  1. The "PRESENT" bit becomes a valid bit. Using LOADALL, you may load a descriptor cache register whose P bit is marked not present (P=0). During normal CPU operaion, simply loading the segment selector with a descriptor table entry whose P=0 will cause an exception-11. This is different that operating with LOADALL. LOADALL will let you load the descriptor cache register with P=0. But any memory reference using that segment selector will cause exception-13.
  2. The DPL field for SS & CS descriptors determine the CPL.
  3. The DPL field for DS, ES, FS, & GS should be 3.
  4. The Granularity (G) bit has no effect on the limit field in the descriptor cache register
  5. A Code segment (CS) may be Read/Write/Executable by setting the access rights as a Read/Write/Data segment. This will even work in protected mode.
; A closer look at the access rights field definitions:
;  2 2 2 2 1 1 1 1 1 1 1  Bit     2 2 2 2 1 1 1 1 1 1
;  3 2 1 0 9 8 7 6 5 4 3  Offset  3 2 1 0 9 8 7 6 5 4
; +-+---+-+-----+-+-+-+-+        +-+---+-+-----+-+-+-+
; |P|DPL|S|Type |A|0|G|D|        |P|DPL|S| Type  |G|D|
; | | | |0| | | | | | | |        | | | |1| | | | | | |
; +-+---+-+-----+-+-+-+-+        +-+---+-+-----+-+-+-+
; Bit:
; P     Present bit.  1=Present, 0=Not present.
;         This bit signals the CPU if the segment addressed by the
;         segment base address is actually present in memory.
; DPL   Descriptor Privilege Level:  0=highest, 3=lowest
; S     System descriptor:  0=Code, Data; 1=System descriptor
; Type  Segment Type:  (S=0)
;         +-+-+-+
;         |X|Y|Z|
;         +-+-+-+
;          | | |
;          | | +-- Read/Write            0=Read-only  1=Read/Write
;          | +---- Expansion direction.  0=Expand up  1=Expand down
;          +------ Executable            0=Data Seg   1=Code Seg
; Type  Segment Type:  (S=1)
;         0000 = Reserved
;         0001 = Available 286 TSS
;         0010 = LDT
;         0011 = Busy 286 TSS
;         0100 = 286 Call Gate
;         0101 = Task Gate
;         0110 = 286 Interrupt Gate
;         0111 = 286 Trap Gate
;         1000 = Reserved
;         1001 = Available 386, 486 TSS
;         1010 = Reserved
;         1011 = Busy 386, 486 TSS
;         1100 = 386, 486 Call Gate
;         1101 = Reserved
;         1110 = 386, 486 Interrupt Gate
;         1111 = 386, 486 Trap Gate
; A     Accessed  (S=0)          0=Not Accessed 1=Accessed
;         The processor sets this bit when the descriptor is
;         accessed.
; G     Granularity                      0=Byte 1=4k
;         When set, upon loading the limit field of the descriptor
;         cache register, the CPU shifts the limit by 12, and fills
;         in the 1st 12 bits with 1's as follows:
;           SHL   LIMIT,12
;           OR    LIMIT,0FFFh
; D     Default operand size             0=16-bit       1=32-bit
;         When set, the CPU interprets all operands, and effective
;         addresses as 32-bit values.  When clear, all operands
;         and effective addresses are 16-bit values.  This bit
;         is only applicable to the CS descriptor cache.

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