Question

我正在尝试在MIPS中实现puts。我有一个程序putchar，它在屏幕上打印一个字符：

    .text
putchar:
    lui $t0, 0xffff     # base address of memory map
XReady:
    lw $t1, 8($t0)      #read from transmitter control register
    andi $t1, $t1, 0x1  # extract ready bit
    beqz $t1, XReady       # if 1, store char ; else, loop
    sw $a0, 12($t0)     # send character to display
    jr $ra                 # return to place in program before function call

在我的主子程序中，我将$ a0设置为我要打印的字符串，然后调用puts：

la $a0, array       # defined in .data as -- array: .space 2000 --
gal puts

我在puts第242行的字边界上找不到错误提取地址（标有*）

    .text
puts:
    addi $sp, $sp, -24  # make room for 6 registers
    sw $ra, 20($sp)     # save $ra on the stack
    sw $s0, 16($sp)     # save $s0 on the stack
    sw, $s1, 12($sp)    # save $s1 on the stack
    sw, $s2, 8($sp)     # save $s2 on the stack
    sw, $s3, 4($sp)     # save $s3 on the stack
    sw, $s4, 0($sp)     # save $s4 on the stack

    move $s0, $a0       # copy parameter $a0 into $s0
    move $s1, $a1       # copy parameter $a0 into $s1
    move $s2, $a2       # copy parameter $a0 into $s2
    move $s3, $a3       # copy parameter $a0 into $s3
    move $s4, $zero     # s4 is a character counter. $s4 = 0
getsLoop:
    addi $t0, $zero, 0x00   # Put NULL ascii character inside $t0
    sll $t1, $s4, 2     # create buffer storing address ($t1 = $s1 * 4)
    add $t2, $s0, $t1   # register #t2 now holds buffer address
*   lw $t3, ($s0)       # load char into #t3
    beq $t3, $t0, exitPuts  # exit puts if the current character is the NULL character

    move $a0, $t3       # put the character to print inside $a0, accessible by putchar
    jal putchar     # print char using putchar
    addi $s0, $s0, 1    # character count += 1
    j getsLoop      # Loop to print next character
exitPuts:
    lw $s4, 0($sp)      # restore stack
    lw $s3, 4($sp)      # -
    lw $s2, 8($sp)      # -
    lw $s1, 12($sp)     # -
    lw $s0, 16($sp)     # -
    lw $ra, 20($sp)     # -
    addi $sp, $sp, 20   # pop from stack
    jr $ra          # return

我不知道为什么我收到此错误... $s0是否在{main}中定义了array的地址？

Answer 1

当您使用lw时，您会加载一个完整的字（4个字节，不要与自然语言中的单词混淆），并且该单词必须在单词边界上对齐（地址的两个最低有效位必须为0）。

ASCII字符通常使用每个字符一个字节存储，因此您应使用lbu指令加载它们而不是lw。

MIPS - 0x004001e8处的运行时异常：提取地址未在字边界0x10010001上对齐

1 个答案: