[Qemu-devel] [MIPS Malta] Wrong relative jump in the YAMON print subroutine

[Jakub Jermar]

Hello,

there appears to be a bug in the hand-written machine code which causes
the YAMON print subroutine to jump to a wrong location after printing
the first character. In hw/mips_malta.c, line 619, there is:

stl_raw(p++, 0x08000205);                                    /* j 814 */

which results in the following wrong code being generated:

   0xbfc00814:  lbu     a0,0(t2)
   0xbfc00818:  addiu   t2,t2,1
   0xbfc0081c:  beqz    a0,0xbfc00834
   0xbfc00820:  nop
   0xbfc00824:  jal     0xbfc00870
   0xbfc00828:  nop
   0xbfc0082c:  j       0xb0000814     <==== HERE

While in fact we would like it to branch to 0xbfc00814 instead. To
achieve this effect, the line needs to be changed to read as:

stl_raw(p++, 0x1000fff9);                                   /* j 814 */

so that we get:

   0xbfc00814:  lbu     a0,0(t2)
   0xbfc00818:  addiu   t2,t2,1
   0xbfc0081c:  beqz    a0,0xbfc00834
   0xbfc00820:  nop
   0xbfc00824:  jal     0xbfc00870
   0xbfc00828:  nop
   0xbfc0082c:  b       0xbfc00814

I verified the print subroutine works as expected with the fix.

Please find the fix attached to this message.

Regards,
Jakub

yamon-print.patch
Description: Text Data