在＆t语法中减去汇编代码中的“char”

Question

我试图找出一个汇编示例，它在& t语法。代码是找到字符串的小写字母。在第26和第28行，距离是什么意思？我知道这里的$'a'有点像ascii，但距离是多少？为什么我们不把$ b1与'a'和'z'进行比较以查看它是否是小写？

1 .data 
2 x: .string "c92jemc82ne<824j8vcm92jq3.,.u" 
3 counts: 
4 .rept 26 
5 .byte 0 
6 .endr 
7 .text 
8 .globl _start 
9 _start: 
10 # EAX will always point to the current character to be tallied 
11 movl $x, %eax 
12 top: 
13 # need to zero out all of EBX for later use (see subl) 
14 movl $0, %ebx 
15 # get the character to be tallied 
16 movb (%eax), %bl 
17 # check for end of string 
18 cmpb $0, %bl 
19 jz done 
20 # check to see if in range ’a’-’z’ 
21 cmpb $’a’, %bl 
22 js nextchar 
23 cmpb $’z’+1, %bl 
24 jge nextchar 
25 # find distance past counts where we will increment 
26 subl $’a’,%ebx 
27 # add that distance to counts to get address of place to increment 
28 addl $counts, %ebx 
29 # now increment 
30 incb (%ebx) 
31 # OK, ready to go to the next character in the string 
32 nextchar: 
33 addl $1, %eax 
34 jmp top 
35 done: movl %edx, %edx` 

Answer 1

我知道$ '一' 这里是类似ASCII，但什么是距离？

不 “像”，它是'a' ASCII码。这里的距离表示距离a ASCII代码有多远，即从“a”代码中减去其代码。

为什么我们只是将$ b1与'a'和'z'进行比较以查看它是否为小写？

他们已经这样做：

20 # check to see if in range ’a’-’z’ 
21 cmpb $’a’, %bl 
22 js nextchar 
23 cmpb $’z’+1, %bl 
24 jge nextchar