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15.6 Differences From 8086

In general, V86 mode will correctly execute software designed for the 8086, 8088, 80186, and 80188. Following is a list of the minor differences between 8086 execution on the 80386 and on an 8086.
  1. Instruction clock counts. The 80386 takes fewer clocks for most instructions than the 8086/8088. The areas most likely to be affected are:
    • Delays required by I/O devices between I/O operations.
    • Assumed delays with 8086/8088 operating in parallel with an 8087.
  2. Divide exceptions point to the DIV instruction. Divide exceptions on the 80386 always leave the saved CS:IP value pointing to the instruction that failed. On the 8086/8088, the CS:IP value points to the next instruction.
  3. Undefined 8086/8088 opcodes. Opcodes that were not defined for the 8086/8088 will cause exception 6 or will execute one of the new instructions defined for the 80386.
  4. Value written by PUSH SP. The 80386 pushes a different value on the stack for PUSH SP than the 8086/8088. The 80386 pushes the value of SP before SP is incremented as part of the push operation; the 8086/8088 pushes the value of SP after it is incremented. If the value pushed is important, replace PUSH SP instructions with the following three instructions:
    PUSH  BP
    MOV   BP, SP
    XCHG  BP, [BP]
    
    This code functions as the 8086/8088 PUSH SP instruction on the 80386.
  5. Shift or rotate by more than 31 bits. The 80386 masks all shift and rotate counts to the low-order five bits. This MOD 32 operation limits the count to a maximum of 31 bits, thereby limiting the time that interrupt response is delayed while the instruction is executing.
  6. Redundant prefixes. The 80386 sets a limit of 15 bytes on instruction length. The only way to violate this limit is by putting redundant prefixes before an instruction. Exception 13 occurs if the limit on instruction length is violated. The 8086/8088 has no instruction length limit.
  7. Operand crossing offset 0 or 65,535. On the 8086, an attempt to access a memory operand that crosses offset 65,535 (e.g., MOV a word to offset 65,535) or offset 0 (e.g., PUSH a word when SP = 1) causes the offset to wrap around modulo 65,536. The 80386 raises an exception in these -- 13 if the segment is a data segment (i.e., if CS, DS, ES, FS, or GS is being used to address the segment), exception 12 if the segment is a stack segment (i.e., if SS is being used).
  8. Sequential execution across offset 65,535. On the 8086, if sequential execution of instructions proceeds past offset 65,535, the processor fetches the next instruction byte from offset 0 of the same segment. On the 80386, the processor raises exception 13 in such a case.
  9. LOCK is restricted to certain instructions. The LOCK prefix and its corresponding output signal should only be used to prevent other bus masters from interrupting a data movement operation. The 80386 always asserts the LOCK signal during an XCHG instruction with memory (even if the LOCK prefix is not used). LOCK may only be used with the following 80386 instructions when they update memory: BTS, BTR, BTC, XCHG, ADD, ADC, SUB, SBB, INC, DEC, AND, OR, XOR, NOT, and NEG. An undefined-opcode exception (interrupt 6) results from using LOCK before any other instruction.
  10. Single-stepping external interrupt handlers. The priority of the 80386 single-step exception is different from that of the 8086/8088. The change prevents an external interrupt handler from being single-stepped if the interrupt occurs while a program is being single-stepped. The 80386 single-step exception has higher priority that any external interrupt. The 80386 will still single-step through an interrupt handler invoked by the INT instructions or by an exception.
  11. IDIV exceptions for quotients of 80H or 8000H. The 80386 can generate the largest negative number as a quotient for the IDIV instruction. The 8086/8088 causes exception zero instead.
  12. Flags in stack. The setting of the flags stored by PUSHF, by interrupts, and by exceptions is different from that stored by the 8086 in bit positions 12 through 15. On the 8086 these bits are stored as ones, but in V86 mode bit 15 is always zero, and bits 14 through 12 reflect the last value loaded into them.
  13. NMI interrupting NMI handlers. After an NMI is recognized on the 80386, the NMI interrupt is masked until an IRET instruction is executed.
  14. Coprocessor errors vector to interrupt 16. Any 80386 system with a coprocessor must use interrupt vector 16 for the coprocessor error exception. If an 8086/8088 system uses another vector for the 8087 interrupt, both vectors should point to the coprocessor-error exception handler.
  15. Numeric exception handlers should allow prefixes. On the 80386, the value of CS:IP saved for coprocessor exceptions points at any prefixes before an ESC instruction. On 8086/8088 systems, the saved CS:IP points to the ESC instruction itself.
  16. Coprocessor does not use interrupt controller. The coprocessor error signal to the 80386 does not pass through an interrupt controller (an 8087 INT signal does). Some instructions in a coprocessor error handler may need to be deleted if they deal with the interrupt controller.


up: Chapter 15 -- Virtual 8086 Mode
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next: 15.7 Differences From 80286 Real-Address Mode