最近编辑
我试图在x86 MASM上运行这个浮点二次方程式程序。这段代码可以在Kip Irvine x86教科书中找到，我想看看它是如何工作的。以下代码如下：
include irvine32.inc
.DATA
a REAL4 3.0
b REAL4 7.0
cc REAL4 2.0
posx REAL4 0.0
negx REAL4 0.0
.CODE
main proc
; Solve quadratic equation - no error checking
; The formula is: -b +/- squareroot(b2 - 4ac) / (2a)
fld1 ; Get constants 2 and 4
fadd st,st ; 2 at bottom
fld st ; Copy it
fmul a ; = 2a
fmul st(1),st ; = 4a
fxch ; Exchange
fmul cc ; = 4ac
fld b ; Load b
fmul st,st ; = b2
fsubr ; = b2 - 4ac
; Negative value here produces error
fsqrt ; = square root(b2 - 4ac)
fld b ; Load b
fchs ; Make it negative
fxch ; Exchange
fld st ; Copy square root
fadd st,st(2) ; Plus version = -b + root(b2 - 4ac)
fxch ; Exchange
fsubp st(2),st ; Minus version = -b - root(b2 - 4ac)
fdiv st,st(2) ; Divide plus version
fstp posx ; Store it
fdivr ; Divide minus version
fstp negx ; Store it
call writeint
exit
main endp
end main
所以我能够让我的程序完全编译，执行和工作。但是，每当我运行程序时，我都会得到这个结果：
+1694175115
为什么结果如此之大？我也尝试调用writefloat，但它说这个程序不在Irvine32.inc或Macros.inc库中。有人能告诉我为什么它不起作用，需要修复什么？谢谢。
答案 0 :(得分：5)
浮点数在特殊处理器（FPU）的特殊寄存器中处理，并以特殊格式存储，不能视为整数（WriteInt）。浮点数包含有关数字的更多信息，如符号和指数。数字本身通过适当的指数更改为1到2之间的数字，其中前导1通常是隐藏的。看看这里的双重格式：https://en.wikipedia.org/wiki/Double-precision_floating-point_format。这些数字不太准确。
至少自11年以来，Irvine32库提供函数WriteFloat以指数形式显示FPU寄存器ST0的值。它不会弹出或释放该注册表。
更改
call writeint
到
fld posx ; Load floating point number into ST0
call WriteFloat ; Write ST0
ffree st[0] ; Free ST0 - don't forget it!
call Crlf ; New line
fld negx ; Load floating point number into ST0
call WriteFloat ; Write ST0
ffree st[0] ; Free ST0 - don't forget it!
call Crlf ; New line
如果您的图书馆没有WriteFloat我建议从Irvine的主页下载并安装最新文件：http://www.kipirvine.com/asm/examples/index.htm（第七版的示例程序和链接库源代码）。您还可以使用其他库，例如C运行时库（msvcrt.inc和msvcrt.lib）或Raymond Filiatreault's FPU library。
如果您不能使用提供浮点例程的库，则必须自己转换数字：
INCLUDE irvine32.inc
.DATA
a REAL4 3.0
b REAL4 7.0
cc REAL4 2.0
posx REAL4 0.0
negx REAL4 0.0
buf BYTE 1024 DUP (?)
.CODE
double2dec PROC C USES edi ; Args: ST(0): FPU-register to convert, EDI: pointer to string
LOCAL CONTROL_WORD:WORD, TEN:WORD, TEMP:WORD, DUMMY:QWORD
; modifying rounding mode
fstcw CONTROL_WORD
mov ax, CONTROL_WORD
or ah, 00001100b ; Set RC=11: truncating rounding mode
mov TEMP, ax
fldcw TEMP ; Load new rounding mode
; Check for negative
ftst ; ST0 negative?
fstsw ax
test ah, 001b
jz @F ; No: skip the next instructions
mov byte ptr [edi], '-' ; Negative sign
add edi, 1
@@:
FABS ; Abs (upper case to differ from C-library)
; Separate integer and fractional part & convert integer part into ASCII
fst st(1) ; Doubling ST(0) - ST(1)=ST(0)
frndint ; ST(0) to integer
fsub st(1), st(0) ; Integral part in ST(0), fractional part in ST(1)
; Move 10 to st(1)
mov TEN, 10
fild TEN
fxch
xor ecx, ecx ; Push/pop counter
@@: ; First loop
fst st(3) ; Preserve ST(0)
fprem ; ST(0) = remainder ST(0)/ST(1)
fistp word ptr TEMP ; ST(3) -> ST(2) !
push word ptr TEMP
inc ecx
fld st(2) ; Restore ST(0)
fdiv st(0), st(1)
frndint ; ST(0) to integer
fxam ; ST0 == 0.0?
fstsw ax
sahf
jnz @B ; No: loop
fxch st(2) ; ST(0) <-> ST(2) (fractional part)
ffree st(2)
ffree st(3)
@@: ; Second loop
pop ax
or al, '0'
mov [edi], al
inc edi
loop @B ; Loop ECX times
mov byte ptr [edi], '.' ; Decimal point
add edi, 1
; Isolate digits of fractional part and store ASCII
get_fractional:
fmul st(0), st(1) ; Multiply by 10 (shift one decimal digit into integer part)
fist word ptr TEMP ; Store digit
fisub word ptr TEMP ; Clear integer part
mov al, byte ptr TEMP ; Load digit
or al, 30h ; Convert digit to ASCII
mov byte ptr [edi], al ; Append it to string
add edi, 1 ; Increment pointer to string
fxam ; ST0 == 0.0?
fstsw ax
sahf
jnz get_fractional ; No: once more
mov byte ptr [edi], 0 ; Null-termination for ASCIIZ
; clean up FPU
ffree st(0) ; Empty ST(0)
ffree st(1) ; Empty ST(1)
fldcw CONTROL_WORD ; Restore old rounding mode
ret ; Return: EDI points to the null-terminated string
double2dec ENDP
main proc
; Solve quadratic equation - no error checking
; The formula is: -b +/- squareroot(b2 - 4ac) / (2a)
fld1 ; Get constants 2 and 4
fadd st,st ; 2 at bottom
fld st ; Copy it
fmul a ; = 2a
fmul st(1),st ; = 4a
fxch ; Exchange
fmul cc ; = 4ac
fld b ; Load b
fmul st,st ; = b2
fsubr ; = b2 - 4ac
; Negative value here produces error
fsqrt ; = square root(b2 - 4ac)
fld b ; Load b
fchs ; Make it negative
fxch ; Exchange
fld st ; Copy square root
fadd st,st(2) ; Plus version = -b + root(b2 - 4ac)
fxch ; Exchange
fsubp st(2),st ; Minus version = -b - root(b2 - 4ac)
fdiv st,st(2) ; Divide plus version
fstp posx ; Store it
fdivr ; Divide minus version
fstp negx ; Store it
; Write the results
fld posx ; Load floating point number into ST0
lea edi, buf ; EDI: pointer to a buffer for a string
call double2dec ; Convert ST0 to buf and pop
mov edx, edi ; EDX: pointer to a null-terminated string
call WriteString ; Irvine32
call Crlf ; Irvine32: New line
fld negx ; Load floating point number into ST0
lea edi, buf ; EDI: pointer to a buffer for a string
call double2dec ; Convert ST0 to buf and pop
mov edx, edi ; EDX: pointer to a null-terminated string
call WriteString ; Irvine32
call Crlf ; Irvine32: New line
exit ; Irvine32: ExitProcess
main ENDP
end main
答案 1 :(得分：4)
感谢Michael Petch
你的错误不在于计算本身，它在MASM中是正确的，但在打印结果时。对于打印浮点数，writeint不正确;你应该使用WriteFloat，它有自己的调用约定。
WriteFloat接受st(0)中的单个浮点数并将其打印到控制台 [1] 。它不弹出x87堆栈中的值。
因此，在您的FPU代码之后，您应该立即添加
fld posx
call WriteFloat
call Crlf
fld negx
call WriteFloat
call Crlf
emms
您还应该在开头包含正确的MASM。类似于：
INCLUDE irvine32.inc
INCLUDE floatio.inc
INCLUDE macros.inc
INCLUDELIB kernel32.lib
INCLUDELIB user32.lib
INCLUDELIB Irvine32.lib
在我的机器上缺少MASM，我将程序重写为具有内联汇编的GNU C程序，并且除了每个指令在那一刻放置了浮点堆栈的状态。
#include <stdio.h>
int main(void){
asm(
".intel_syntax\n"
".data\n"
"a: .single 3.0\n"
"b: .single 7.0\n"
"cc: .single 2.0\n"
"posx: .single 0.0\n"
"negx: .single 0.0\n"
".text\n"
"fld1\n" // [1]
"fadd %st, %st\n" // [2]
"fld %st\n" // [2, 2]
"fmul dword ptr a\n" // [2a, 2]
"fmul %st(1), %st\n" // [2a, 4a]
"fxch\n" // [4a, 2a]
"fmul dword ptr cc\n" // [4ac, 2a]
"fld dword ptr b\n" // [b, 4ac, 2a]
"fmul %st, %st\n" // [b^2, 4ac, 2a]
"fsubrp\n" // [b^2-4ac, 2a]
"fsqrt\n" // [sqrt(b^2-4ac), 2a]
"fld dword ptr b\n" // [b, sqrt(b^2-4ac), 2a]
"fchs\n" // [-b, sqrt(b^2-4ac), 2a]
"fxch\n" // [sqrt(b^2-4ac), -b, 2a]
"fld %st\n" // [sqrt(b^2-4ac), sqrt(b^2-4ac), -b, 2a]
"fadd %st, %st(2)\n" // [-b+sqrt(b^2-4ac), sqrt(b^2-4ac), -b, 2a]
"fxch\n" // [ sqrt(b^2-4ac), -b+sqrt(b^2-4ac), -b, 2a]
"fsubp %st(2), %st\n" // [-b+sqrt(b^2-4ac), -b-sqrt(b^2-4ac), 2a]
"fdiv %st, %st(2)\n" // [(-b+sqrt(b^2-4ac))/2a, -b-sqrt(b^2-4ac), 2a]
"fstp dword ptr posx\n" // [ -b-sqrt(b^2-4ac), 2a]
"fdivrp\n" // [(-b-sqrt(b^2-4ac))/2a]
"fstp dword ptr negx\n" // []
".att_syntax\n"
);
extern float posx, negx;
printf("posx: %+0.17f\nnegx: %+0.17f\n", posx, negx);
return 0;
}
打印
posx: -0.33333334326744080
negx: -2.00000000000000000
哪个是对的：
[1] §12.2.7读取和写入浮点值