Best way to simulate old, slow processor on modern hardware?

Question

I really like the idea of running, optimizing my software on old hardware, because you can viscerally feel when things are slower (or faster!). The most obvious way to do this is to buy an old system and literally use it for development, but that would allow down my IDE, and compiler and all other development tasks, which is less helpful, and (possibly) unnecessary.

I want to be able to:

• Run my application at various levels of performance, on demand
• At the same time, run my IDE, debugger, compiler at full speed
• On a single system

Nice to have:

• Simulate real, specific old systems, with some accuracy
• Similarly throttle memory speed, and size
• Optionally run my build system slowly

Answer 1

Try use QEMU in full emulation mode, but keep in mind it's use more cpu resources.

https://stuff.mit.edu/afs/sipb/project/phone-project/OldFiles/share/doc/qemu/qemu-doc.html

QEMU has two operating modes:

• Full system emulation. In this mode, QEMU emulates a full system (for example a PC), including one or several processors and various peripherals. It can be used to launch different Operating Systems without rebooting the PC or to debug system code.

• User mode emulation (Linux host only). In this mode, QEMU can launch Linux processes compiled for one CPU on another CPU.

Possible architectures can see there: https://wiki.qemu.org/Documentation/Platforms