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linux中fstp命令的简单介绍( 三 )

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bigger than 32 bits are stored in little-endian form, i.e. with the least significant DWORD at the
lowest address, and DWORD aligned.
Function return values are passed in registers in most cases. 8-bit integers are returned in
AL, 16-bit integers in AX, 32-bit integers, pointers, and Booleans in EAX, 64-bit integers in
EDX:EAX, and floating-point values in ST(0). Structures and class objects not exceeding
64 bits size are returned in the same way as integers, even if the structure contains floating
point values. Structures and class objects bigger than 64 bits are returned through a pointer
passed to the function as the first parameter and returned in EAX. Compilers that don\'t
support 64-bit integers may return structures bigger than 32 bits through a pointer. The
Borland compiler also returns structures through a pointer if the size is not a power of 2.
Registers EAX, ECX and EDX may be changed by a procedure. All other general-purpose
registers (EBX, ESI, EDI, EBP) must be saved and restored if they are used. The value of
ESP must be divisible by 4 at all times, so don\'t push 16-bit data on the stack. Segment
registers cannot be changed, not even temporarily. CS, DS, ES, and SS all point to the flat
segment group. FS is used for a thread environment block. GS is unused, but reserved.
Flags may be changed by a procedure with the following restrictions: The direction flag is 0
by default. The direction flag may be set temporarily, but must be cleared before any call or
return. The interrupt flag cannot be cleared. The floating-point register stack is empty at the
entry of a procedure and must be empty at return, except for ST(0) if it is used for return
value. MMX registers may be changed by the procedure and if so cleared by EMMS before
returning and before calling any other procedure that may use floating-point registers. All
XMM registers can be modified by procedures. Rules for passing parameters and return
values in XMM registers are described in Intel\'s application note AP 589 "Software
Conventions for Streaming SIMD Extensions". A procedure can rely on EBX, ESI, EDI, EBP
and all segment registers being unchanged across a call to another procedure.
2. Register usage in Linux
The rules for register usage in Linux appear to be almost the same as for 32-bit windows.
Registers EAX, ECX, and EDX may be changed by a procedure. All other general-purpose
registers must be saved. There appears to be no rule for the direction flag. Function return
values are transferred in the same way as under Windows. Calling conventions are the
same, except for the fact that no underscore is prefixed to public names. I have no
information about the use of FS and GS in Linux. It is not difficult to make an assembly
function that works under both Windows and Linux, if only you take these minor differences
into account.
八、位操作指令，处理器控制指令
1.位操作指令，8086新增的一组指令，包括位测试，位扫描 。BT,BTC,BTR,BTS,BSF,BSR
1.1 BT(Bit Test)，位测试指令，指令格式:
BT OPRD1,OPRD2,规则：操作作OPRD1可以是16位或32位的通用寄存器或者存储单元 。操作数OPRD2必须是8位立即数或者是与OPRD1操作数长度相等的通用寄存器 。如果用OPRD2除以OPRD1，假设商存放在Divd中，余数存放在Mod中，那么对OPRD1操作数要进行测试的位号就是Mod,它的主要功能就是把要测试位的值送往CF，看几个简单的例子：
1.2 BTC(Bit Test And Complement)，测试并取反用法和规则与BT是一样，但在功能有些不同，它不但将要测试位的值送往CF，并且还将该位取反 。
1.3 BTR(Bit Test And Reset)，测试并复位，用法和规则与BT是一样，但在功能有些不同，它不但将要测试位的值送往CF，并且还将该位复位(即清0) 。

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