QEMU is an open-source emulator and virtualization tool. It can emulates several processor architectures including x86 (Pentium 100 too).
How QEMU works?
QEMU translates instructions (dynamic binary translation - DBT): Blocks of binary code are translated from the source to the target instruction set
Memory, I/O ports and other peripheries are emulated. This includes also: graphics card, disks, keyboard and other devices
An open-source Virtual BIOS is used, like SeaBIOS. This BIOS initializes and manages virtual hardware, which is similar to a real PC with Pentium 100
Speed and accuracy: QEMU does not typically emulate the Pentium 100 processor with maximum cycle-accurate emulation, but rather focuses on functional emulation, trying to emulate instruction and memory behavior as accurately as possible. This makes QEMU more suitable for running legacy systems and applications than for researching timing at the cycle level.