Question

编译汇编代码时收到错误。这个程序的目的是接收一个字符串和一个字符。然后找到指定字符的频率。

编辑：我遇到了一个关于字符频率计数的新问题，但我不知道我应该采取哪些步骤来调试它。这是我得到的错误。

 Exception 14 at EIP=0010021e: Page fault

.data                           #Used for declaring initialized data or constants
.bss                            #Used for declaring variables
.text                           #This tells the kernel where the program execution begins
.globl _count                   #Allows the function to be called externally

_count:
    pushl    %ebp
    mov      %ebp, %esp

    movl     $0,        %eax    # ensure return register is set to 0
    movb     8(%ebp),   %esi    # move address of string pointer into esi
    movb     12(%ebp),  %ch     # move compare character into ch

    while:
        movb        (%esi), %cl       # move character of interest in string by index into cl
        cmp     $0, %cl         # compare current character against null terminator
        je      exit            # hopefully we weren't looking to count that character :)
        inc     %esi            # then increment esi for next iteration of loop
        cmp     %cl, %ch        # compare string character against desired character
        jne     while           # if not the same, return back to the start of the loop
        inc     %eax            # increment counter by one if the characters are the same
        jmp     while           # return back to the start of the loop
    exit:
        popl     %ebp           # pop off ebp, return value is in eax
        ret
.end

这是用i386编写的。

extern int count(char *string, char c);

int main(void)
{
  char s[100];
  char c;
  printf("Enter a string of characters:\n");
  scanf("%s", s);
  printf("Enter a character to count:\n");
  scanf("%c", &c);
  printf("\nThe number of %c's in the string %s is %d\n", c, s, count(s, c));
  return 0;
}

生成文件：

# makefile: makefile for assembly-language examples
# 
# Assumes a matched pair of sources, e.g., myprog.s and myprogc.c,
# an assembler source for a function and its C driver.

# Usage: For an assembler source, say myprog.s, with a corresponding 
#  C driver myprogc.c, set A to the basename in the make command line: 
#  "make A=myprog myprog.lnx" or just "make A=myprog"
#  (since $(A).lnx is the first make target)
#  Then myprog.lnx is ready for downloading via "mtip -f myprog.lnx"
#  (followed by ~r and ~d inside the mtip program)

# also "make clean" to clean up non-source files in a directory

# Here the assembler program has the primary source name,
# the C program is just a driver of almost the same basename:
C = $(A)c

# system directories needed for compilers, libraries, header files--
# assumes the environment variables SAPC_TOOLS, SAPC_GNUBIN, and SAPC_SRC
# are set up, usually by the ulab module

PC_LIB = $(SAPC_TOOLS)/lib
PC_INC = $(SAPC_TOOLS)/include

CC   = $(SAPC_GNUBIN)/i386-gcc
CFLAGS = -g -Wall -Wno-implicit -Wshadow -I$(PC_INC)
AS   = $(SAPC_GNUBIN)/i386-as
LD   = $(SAPC_GNUBIN)/i386-ld
NM   = $(SAPC_GNUBIN)/i386-nm

# File suffixes:
# .c    C source (often useful both for UNIX host and SAPC)
# .s    assembly language source
# .opc  relocatable machine code, initialized data, etc., for SA PC
# .lnx  executable image (bits as in memory), for SA PC (Linux a.out format)
# .syms text file of .lnx's symbols and their values (the "symbol table")
# Symbol file "syms"--for most recently built executable in directory

# PC executable--tell ld to start code at 0x1000e0, load special startup
# module, special PC C libraries--
# Code has 0x20 byte header, so use "go 100100" (hopefully easier to
# remember than 100020)

$(A).lnx: $(A).opc $(C).opc 
    $(LD) -N -Ttext 1000e0 -o $(A).lnx \
        $(PC_LIB)/startup0.opc $(PC_LIB)/startup.opc \
        $(C).opc $(A).opc $(PC_LIB)/libc.a
    rm -f syms;$(NM) -n $(A).lnx>$(A).syms;ln -s $(A).syms syms

$(A).opc: $(A).s
    $(AS) -o $(A).opc $(A).s

# tell gcc to use $(PC_INC) for #include <...> headers--
$(C).opc: $(C).c
    $(CC) $(CFLAGS) -c -o $(C).opc $(C).c

clean:
    rm -f *.o *.opc *.syms *.lnx core syms

汇编代码问题

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