17 9 mixing 16 bit and 32 bit code 17 10 18 intel

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Unformatted text preview: the stack-segment descriptor currently in use (that is, when B is clear, SP is used, and when B is set, ESP is used). An unmodified 16-bit code segment that has run successfully on an 8086 processor or in real-mode on a P6-family processor will have its D flag clear and will not use operand-size override prefixes. As a result, all CALL instructions in this code segment will use the 16-bit operandsize attribute. Procedures in these code segments can be modified to safely call procedures to 32-bit code segments in either of two ways: • • Relink the CALL instruction to point to 32-bit call gates (refer to Section, “Passing Parameters With a Gate”). Add a 32-bit operand-size prefix to each CALL instruction. PASSING PARAMETERS WITH A GATE When referencing 32-bit gates with 16-bit procedures, it is important to consider the number of parameters passed in each procedure call. The count field of the gate descriptor specifies the size of the parameter string to copy from the current stack to the stack of a more privileged (numerically lower privilege level) procedure. The count field of a 16-bit gate specifies the number of 16-bit words to be copied, whereas the count field of a 32-bit gate specifies the number of 32-bit doublewords to be copied. The count field for a 32-bit gate must thus be half the size of the number of words being placed on the stack by a 16-bit procedure. Also, the 16-bit procedure must use an even number of words as parameters. 17-7 MIXING 16-BIT AND 32-BIT CODE 17.4.3. Interrupt Control Transfers A program-control transfer caused by an exception or interrupt is always carried out through an interrupt or trap gate (located in the IDT). Here, the type of the gate (16-bit or 32-bit) determines the operand-size attribute used in the implicit call to the exception or interrupt handler procedure in another code segment. A 32-bit interrupt or trap gate provides a safe interface to a 32-bit exception or interrupt handler when the exception or interrupt occurs in either a 32-bit or a 16-bit code segment. It is sometimes impractical, however, to place exception or interrupt handlers in 16-bit code segments, because only 16-bit return addresses are saved on the stack. If an exception or interrupt occurs in a 32-bit code segment when the EIP was greater than FFFFH, the 16-bit handler procedure cannot provide the correct return address. 17.4.4. Parameter Translation When segment offsets or pointers (which contain segment offsets) are passed as parameters between 16-bit and 32-bit procedures, some translation is required. If a 32-bit procedure passes a pointer to data located beyond 64 KBytes to a 16-bit procedure, the 16-bit procedure cannot use it. Except for this limitation, interface code can perform any format conversion between 32-bit and 16-bit pointers that may be needed. Parameters passed by value between 32-bit and 16-bit code also may require translation between 32-bit and 16-bit formats. The form of the translation is application-dependent. 17.4.5. Writing Interface Procedures Placing interface code between 32-bit and 16-bit procedures can be the solution to the following interface problems: • • • • Allowing procedures in 16-bit code segments to call procedures with offsets greater than FFFFH in 32-bit code segments. Matching operand-size attributes between companion CALL and RET instructions. Translating parameters (data), including managing parameter strings with a variable count or an odd number of 16-bit words. The possible invalidation of the upper bits of the ESP register. The interface procedure is simplified where these rules are followed. 1. The interface procedure must reside in a 32-bit code segment (the D flag for the codesegment descriptor is set). 2. All procedures that may be called by 16-bit procedures must have offsets not greater than FFFFH. 3. All return addresses saved by 16-bit procedures must have offsets not greater than FFFFH. 17-8 MIXING 16-BIT AND 32-BIT CODE The interface procedure becomes more complex if any of these rules are violated. For example, if a 16-bit procedure calls a 32-bit procedure with an entry point beyond FFFFH, the interface procedure will need to provide the offset to the entry point. The mapping between 16- and 32-bit addresses is only performed automatically when a call gate is used, because the gate descriptor for a call gate contains a 32-bit address. When a call gate is not used, the interface code must provide the 32-bit address. The structure of the interface procedure depends on the types of calls it is going to support, as follows: • Calls from 16-bit procedures to 32-bit procedures. Calls to the interface procedure from a 16-bit code segment are made with 16-bit CALL instructions (by default, because the D flag for the calling code-segment descriptor is clear), and 16-bit operand-size prefixes are used with RET instructions to return from the interface procedure to the calling procedure. Calls from the interface procedure to 32-bit procedures a...
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at University of California, Berkeley.

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