Error understanding of Call printf parameter stack in assembler

EAX, ECX,EDX,EBX can be 32bit,16bit,8bit accessed as follows:

<-------------------EAX------------------------>
|<----------------------|-----------|----------->|
|<---------AX--------->|
|<---AH--->|<---AL--->|

The test code is as follows:

1 . Section. Data2 Output:3. Asciz"Value is:%x\n"4 Val:5.int 06     7 . Section. Text8 . Globl Main9 Main:Ten     NOP One movl $0x12345678,%eax AMOVL $0,%ecx - MOVW%ax,%cx -     PUSHW%cx the PUSHL $output -     Pagerprintf -Addl $6,%esp -      +MOVL $1,%eax -MOVL $0,%EBX +     int$0x80

My computer is a small-format (MSB in the high-level), so the expected output is: 0x5678, but I compiled the result is: 0x2dec5678, 2 more bytes in front, but clearly I press the stack when the use of PUSHW, It turns out that printf was using 4-byte lengths to get the parameters, so although I was only 2 bytes in the press, it was accessed with 4 bytes.

To modify the code:

1 . Section. Data2 Output:3. Asciz"Value is:%x\n"4 Val:5.int 06     7 . Section. Text8 . Globl Main9 Main:Ten     NOP One movl $0x12345678,%eax A movl%eax,%ecx - PUSHL%ecx - PUSHL $output the     Pagerprintf -Addl $8,%esp -  -MOVL $0,%ecx + MOVW%ax,%cx - PUSHL%ecx + PUSHL $output A     Pagerprintf atAddl $8,%esp -      -MOVL $1,%eax -MOVL $0,%EBX -     int$0x80

Expected output:

Value is:12345678
Value is:5678

Result output:

Value is:12345678
Value Is:12

At first thought is the 0x12345678 of the highest byte 0x12 transferred to the CX inside, feel quite strange, obviously use is MOVW, to wrong is also 0x1234, analysis, the original is printf mischief, function call, its return value is usually stored in the eax inside to pass, And the first printf output: value is:12345678 has exactly 0x12,18 characters (with ' \ n '), so after the code executes 15 lines of call, the value of EAX has been modified by the return value of printf, not the 0x12345678 I expected. So when you call library functions in the Assembly, pay special attention to the special purpose of some special registers, especially when these libraries are generated by other high-level languages through the compilation tool.

Error understanding of Call printf parameter stack in assembler