參數(shù)資料
型號(hào): intel386 DX
廠商: Intel Corp.
英文描述: 32-Bit CHMOS Microprocessor With Integrated Memory Management(32位CHMOS 微處理器帶集成存儲(chǔ)管理)
中文描述: 32位CHMOS微處理器集成內(nèi)存管理(32位CHMOS微處理器帶集成存儲(chǔ)管理)
文件頁數(shù): 19/139頁
文件大小: 1587K
代理商: INTEL386 DX
Intel386
TM
DX MICROPROCESSOR
231630–51
Figure 2-9. Addressing Mode Calculations
2.5.4 Differences Between 16 and 32
Bit Addresses
In order to provide software compatibility with the
80286 and the 8086, the Intel386 DX can execute
16-bit instructions in Real and Protected Modes. The
processor determines the size of the instructions it is
executing by examining the D bit in the CS segment
Descriptor. If the D bit is 0 then all operand lengths
and effective addresses are assumed to be 16 bits
long. If the D bit is 1 then the default length for oper-
ands and addresses is 32 bits. In Real Mode the
default size for operands and addresses is 16-bits.
Regardless of the default precision of the operands
or addresses, the Intel386 DX is able to execute ei-
ther 16 or 32-bit instructions. This is specified via the
use of override prefixes. Two prefixes, the
Operand
Size Prefix
and the
Address Length Prefix
, over-
ride the value of the D bit on an individual instruction
basis. These prefixes are automatically added by In-
tel assemblers.
Example: The processor is executing in Real Mode
and the programmer needs to access the EAX regis-
ters. The assembler code for this might be MOV
EAX, 32-bit MEMORYOP, ASM386 Macro Assem-
bler automatically determines that an Operand Size
Prefix is needed and generates it.
Example: The D bit is 0, and the programmer wishes
to use Scaled Index addressing mode to access an
array. The Address Length Prefix allows the use of
MOV DX, TABLE
[
ESI
*
2
]
. The assembler uses an
Address Length Prefix since, with D
e
0, the default
addressing mode is 16-bits.
Example: The D bit is 1, and the program wants to
store a 16-bit quantity. The Operand Length Prefix is
used to specify only a 16-bit value; MOV MEM16,
DX.
19
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