[英]Reading intel assembly 8086 listing file
Here is my listing file for a program. 这是我的程序清单文件。 However, the question I'm asked is the significance of F0 on line 20. I know the first column represents the address offset and the third column represents the instruction, but how do we figure out what the data means here?
但是,我问的问题是第20行上F0的重要性。我知道第一列代表地址偏移量,第三列代表指令,但是我们如何弄清楚数据在这里的含义?
1 ;
2 ;
3 ;
4 section .data
5
6 00000000 0102030405 number: db 1,2,3,4,5
7 00000005 00 sum: db 0
8
9
10 section .text
11 global _start
12
13 _start:
14 00000000 B905000000 keith: mov ecx, 5
15 00000005 B800000000 ron: mov eax, 0
16 0000000A BB[00000000] mov ebx, number
17 0000000F 0203 again: add al, [ebx]
18 00000011 81C301000000 add ebx,1
19 00000017 81E901000000 sub ecx,1
20 0000001D 75F0 jnz again
21 0000001F A2[05000000] mov [sum], al
22
23 00000024 B801000000 mov eax,1
24 00000029 BB00000000 mov ebx,0
25 0000002E CD80 int 80h
Opcode 75
is a short jump . 操作码
75
是一个短距离跳转 。 A short jump is a relative jump that is less than 128 bytes away forward or backward from the instruction pointer (IP) of the instruction that follows the jump instruction. 短跳转是相对跳转,相对跳转距离跟随跳转指令的指令的指令指针 (IP)少于128个字节。 (NOTE: in the standard CPU fetch-execute cycle, an instruction is fetched, and then the IP is incremented to the next instruction before the fetched instruction is executed.)
(注意:在标准CPU提取执行周期中,将提取一条指令,然后在执行该提取的指令之前 ,将IP递增到下一条指令。)
16 0000000A BB[00000000] mov ebx, number
17 0000000F 0203 again: add al, [ebx]
18 00000011 81C301000000 add ebx,1
19 00000017 81E901000000 sub ecx,1
20 0000001D 75F0 jnz again
21 0000001F A2[05000000] mov [sum], al
In this case, your full opcode is: 75F0
. 在这种情况下,您的完整操作码为:
75F0
。 That's a short jump (if Z flag is 0) a distance from the next IP represented by the signed byte value, F0
. 这是一个短距离的跳跃(如果Z标志为0)与由有符号字节值
F0
表示的下一个IP的距离。 The signed value of F0h
, taken as a 2's complement signed byte, is minus 10h
and is computed as: F0h
的有符号值(作为2的补码有符号字节)为负10h
,其计算公式为:
0000000F ; address of "again:" label
- 0000001F ; address of instruction after "jnz"
----------
F0 ; difference in addresses
This tells the CPU to jump back -10h
byte locations from the following IP in order to get to the again:
label. 这告诉CPU 跳回
-10h
从下面的IP字节位置,以获得对again:
标签。
The fact that the distance to jump was so short, the assembler was able to use the short jump . 跳跃距离很短的事实,使组装人员能够使用短距离跳跃 。 Otherwise, if it were greater than 128 bytes away, a different jump opcode would be needed ( eg , near jump if it's within the 64kB segment - otherwise a far jump is needed).
否则,如果距离大于128个字节,则需要一个不同的跳转操作码( 例如 ,如果它在64kB段内,则为近跳转 -否则需要远跳转 )。
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