I386-softmmu

This is the main 'qemu' emulator. This emulates the x86 IBM PC on whatever host Qemu is running on. There are two options to the -M flag to specify a platform:

-M isapc

This will emulate an ISA bus PC.

-M pc

This will emulate a PCI bus based PC. The PCI bus is based on the Intel i440FX / PIIX3 PCI bus/bridge chipset.