Q43142: QuickC: The Inline Assembler Generates 3-Byte JMP Instructions

Article: Q43142
Product(s): See article
Version(s): 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 2-MAY-1989

The following instruction will assemble, link, and run correctly in
the Microsoft Macro Assembler Version 5.10:

   JMP  $+2

However, the Microsoft QuickC Compiler Version 2.00 inline assembler
will generate code that hangs the machine when executed.

This is expected behavior and is due to the fact that the inline
assembler must generate code as assembly mnemonics are encountered.
Therefore, it cannot tell that the instruction is a SHORT jump. The
Microsoft Macro Assembler will optimize the code into a 2-byte jump
instruction while QuickC will generate a 3-byte jump instruction. To
achieve the desired result either change the instruction to the
following:

   JMP  $+3

or

   JMP SHORT $+2

The desired result of this instruction is to simply jump to the
instruction located at the address pointed to by the instruction
pointer, plus the number of bytes specified.

The following MASM program will have the effect of skipping the second
instruction:

                                    TITLE  test
                                    DOSSEG
                                    .MODEL SMALL
                                    .CODE
 0000                           start:
 0000  EB 02                        jmp  $+4
 0002  EB 00                        jmp  $+2
 0004  EB 00                        jmp  $+2
 0006  B8 4C00                      mov  ax,4c00h
 0009  CD 21                        int  21h
                                END start

Clearly, the Assembler is producing a 2-byte JMP instruction. The
following is the code compiled with QuickC Version 2.00 which will
have the same effect:

void main(void_ {
     _asm {
          jmp $+6
          jmp $+3
          jmp $+3
     }

Upon viewing the mixed C/assembly in CodeView 2.20, it is clear that
QuickC is generating a 3-byte JMP instruction:

1:      void main(void) {
/* startup code here */
3:                      jmp $+6
6DD8:002B E90300         JMP       _main+11 (0031)
4:                      jmp $+3
6DD8:002E E90000         JMP       _main+11 (0031)
5:                      jmp $+3
6DD8:0031 E90000         JMP       _main+14 (0034)
7:         }
/* exit code here */

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