2

I'm trying to create a VM of a 486 without FPU, like the 80486SX. I figured this could be accomplished with QEMU. I couldn't find the possibility to directly specify such a cpu (there's only the option -cpu=486). Based on the documentation, I thought it would be possible to remove the FPU by specifying the cpu as follows

qemu-system-i386 -cpu 486,-fpu (...)

But after I installed Debian 5.0 (the last Debian to support the 486), cat/proc tells me that a FPU is indeed considered present (even though it's not in the flags), and gcc will emit floating point instructions for a simple hello-world style program printing doubles, and QEMU will happily execute it.

user@debian-486-nofpu:~/$ cat /proc/cpuinfo 
processor   : 0
vendor_id   : GenuineIntel
cpu family  : 4
model       : 8
model name  : 486 DX/4
stepping    : 0
cache size  : 0 KB
fdiv_bug    : no
hlt_bug     : no
f00f_bug    : no
coma_bug    : no
fpu     : yes
fpu_exception   : no
cpuid level : 1
wp      : yes
flags       : pse
bogomips    : 466.94
clflush size    : 32
power management:
Ant6n
  • 1,887
  • 1
  • 20
  • 26
  • But Linux kernel emulates FPU when it's not present, so it shouldn't make much of a difference for a user running Linux except possibly for speed (though FPU is not much faster than emulation). – FlatAssembler Jun 25 '20 at 09:49

0 Answers0