浮点计划提供无效结果

Question

最近的编辑

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,我得到的结果如下:

+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.

Answer 1

浮点数在特殊处理器(FPU)中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.

至少从11年开始,Irvine32库提供了WriteFloat以指数形式显示FPU寄存器ST0的值的功能.它不会弹出或释放该注册.

更改

call writeint

至

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的FPU库.

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