为什么x86 16位寻址模式没有比例因子，而32位版本却具有比例因子？ [英] Why don't x86 16-bit addressing modes have a scale factor, while the 32-bit version has it?
问题描述
我试图找出x86 16位寻址模式（MASM组件）中不存在比例因子的原因。而32位和64位寻址模式具有比例因子。这是背后的实际原因还是不需要？
I'm trying to figure out a reason for the scale factor not being present in the x86 16-bit addressing modes (MASM assembly). While the 32-bit and 64-bit addressing modes have a scale factor. Is there an actual reasoning behind this or it doesn't need it? I would appreciate it if you could explain.
将不同组件组合起来以创建有效地址的所有可能方式：
All possible ways different components can be combined to create an effective address:
16位和32位寻址之间的差异模式
Differences between 16- and 32-bit addressing modes
推荐答案
16位寻址模式仅允许单个ModRM字节对寄存器（3位），模式（2位）和寄存器/内存操作数（3位）进行编码），因此就没有空间对比例因子进行编码，甚至没有空间让任意寄存器成为基数或索引。 NASM x86 16位寻址模式将它们全部列出，这不是一长串！只是（BP | BX）+（DI | SI）+ disp0 / 8/16 的子集。请记住，在类似 add cx，[bx + si] 的指令中，寄存器目标需要3位 / r
16-bit addressing modes only allow a single ModRM byte to encode the register (3 bits), mode (2 bits) and the register/memory operand (3 bits), so there's no room to encode a scale factor, or even to let arbitrary registers be bases or indices. NASM x86 16-bit addressing modes lists them all, it's not a long list! Just subsets of (BP|BX) + (DI|SI) + disp0/8/16. Remember that in an instruction like add cx, [bx+si], the register destination needs the 3 bit /r field in ModRM to encode which of the 8 GP registers.
（2位模式表示它是寄存器还是存储器，例如添加bx，cx 与添加[bx]，cx ，以及有多少立即移位字节：disp8 / disp16或无移位。）
(The 2-bit "mode" signals whether it's a register or memory, e.g. add bx, cx vs. add [bx], cx, and how many immediate displacement bytes there are: disp8 / disp16 or no displacement.)
在32/64位寻址模式下，ModRM中的r / m字段可以是转义码，表示存在SIB字节（Scale /索引/基数），这为编码具有2个移位计数的缩放索引寻址模式提供了空间。
In 32/64-bit addressing modes, the r/m field in ModRM can be an escape code that signals the presence of a SIB byte (Scale/Index/Base), which gives room to encode scaled-index addressing modes with a 2-bit shift count.
还有足够的编码空间可以让我们使用任何寄存器作为基数，并使用任何寄存器（ESP除外）作为索引。因此32位寻址模式使寄存器更加正交。有关转义序列的详细信息，请参见不允许将rbp用作SIB基础吗？，例如 [esp] 始终需要一个SIB字节，因为这意味着base = ESP的编码是存在SIB字节的转义代码。
And also enough coding space to let us use any register as a base, and any register (except ESP) as an index. So 32-bit addressing modes make the registers more orthogonal. See rbp not allowed as SIB base? for the details on the escape sequences, e.g. [esp] always needs a SIB byte because the encoding that would mean base=ESP is the escape code for the presence of a SIB byte.
请参见 https://wiki.osdev.org/X86 -64_Instruction_Encoding＃32.2F64-bit_addressing_2 或英特尔手册中的ModRM / SIB表以获取更多详细信息。
See https://wiki.osdev.org/X86-64_Instruction_Encoding#32.2F64-bit_addressing_2 or the ModRM/SIB tables in Intel's manuals for more details.
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