Intel IA-32 - Manual (Page 552)

15-2 Vol. 3A
8086 EMULATION
The following is a summary of the core features of the real-address mode execution environment
as would be seen by a program written for the 8086:
•
The processor supports a nominal 1-MByte physical address space (see Section 15.1.1,
“Address Translation in Real-Address Mode”, for specific details). This address space is
divided into segments, each of which can be up to 64 KBytes in length. The base of a
segment is specified with a 16-bit segment selector, which is zero extended to form a
20-bit offset from address 0 in the address space. An operand within a segment is
addressed with a 16-bit offset from the base of the segment. A physical address is thus
formed by adding the offset to the 20-bit segment base (see Section 15.1.1, “Address
Translation in Real-Address Mode”).
•
All operands in “native 8086 code” are 8-bit or 16-bit values. (Operand size override
prefixes can be used to access 32-bit operands.)
•
Eight 16-bit general-purpose registers are provided: AX, BX, CX, DX, SP, BP, SI, and DI.
The extended 32 bit registers (EAX, EBX, ECX, EDX, ESP, EBP, ESI, and EDI) are
accessible to programs that explicitly perform a size override operation.
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Four segment registers are provided: CS, DS, SS, and ES. (The FS and GS registers are
accessible to programs that explicitly access them.) The CS register contains the segment
selector for the code segment; the DS and ES registers contain segment selectors for data
segments; and the SS register contains the segment selector for the stack segment.
•
The 8086 16-bit instruction pointer (IP) is mapped to the lower 16-bits of the EIP register.
Note this register is a 32-bit register and unintentional address wrapping may occur.
•
The 16-bit FLAGS register contains status and control flags. (This register is mapped to
the 16 least significant bits of the 32-bit EFLAGS register.)
•
All of the Intel 8086 instructions are supported (see Section 15.1.3, “Instructions
Supported in Real-Address Mode”).
•
A single, 16-bit-wide stack is provided for handling procedure calls and invocations of
interrupt and exception handlers. This stack is contained in the stack segment identified
with the SS register. The SP (stack pointer) register contains an offset into the stack
segment. The stack grows down (toward lower segment offsets) from the stack pointer.
The BP (base pointer) register also contains an offset into the stack segment that can be
used as a pointer to a parameter list. When a CALL instruction is executed, the processor
pushes the current instruction pointer (the 16 least-significant bits of the EIP register and,
on far calls, the current value of the CS register) onto the stack. On a return, initiated with
a RET instruction, the processor pops the saved instruction pointer from the stack into the
EIP register (and CS register on far returns). When an implicit call to an interrupt or
exception handler is executed, the processor pushes the EIP, CS, and EFLAGS (low-order
16-bits only) registers onto the stack. On a return from an interrupt or exception handler,
initiated with an IRET instruction, the processor pops the saved instruction pointer and
EFLAGS image from the stack into the EIP, CS, and EFLAGS registers.
•
A single interrupt table, called the “interrupt vector table” or “interrupt table,” is
provided for handling interrupts and exceptions (see Figure 15-2). The interrupt table
(which has 4-byte entries) takes the place of the interrupt descriptor table (IDT, with