1. FreeBSD：
FreeBSD 是 FreeBSD 项目提供的免费开源操作系统。它是在两个类 Unix 操作系统 Berkeley Software Distribution (BSD) 和 Research Unix 的基础上设计的。它与 Linux 操作系统非常相似。它是早期著名的操作系统之一，但与其他操作系统相比，现在使用较少。 FreeBSD 的第一个版本于 1993 年推出。FreeBSD 的最新稳定版本是 12.1。
2. OpenVMS：
OpenVMS是由美国数字设备公司(DEC)开发的多用户多处理操作系统。它是一种基于虚拟内存的操作系统，设计用于分时、批处理和事务处理。它可用于许多平台，包括服务器和工作站。它是使用 BLISS、VAX Macro、C、Ada、PL/I、Fortran、UIL、Pascal、SDL、C++ 和 DCL 编写的。 OpenVMS 的第一个版本于 1977 年推出。该操作系统中使用的内核是带有模块的单体。
OpenVMS 和 FreeBSD 的区别：
| OpenVMS | FreeBSD |
|---|---|
| It was developed by Digital Equipment Corporation (DEC). | It was developed and is owned by The FreeBSD Project. |
| It was launched in 1977. | It was launched in 1993. |
| Its target system types are server and workstation. | Its target system type is server, workstation, NAS and embedded systems. |
| Computer architectures supported by OpenVMS are VAX, Alpha and IA-64. | Computer architectures supported are IA-32, x86-64, ARM, MIPS and PowerPC. |
| Its kernel type is Monolithic with modules. | Kernel type is also Monolithic with modules. |
| Its native APIs are Proprietary. | The native APIs are BSD/POSIX. |
| Its package management is PCSI or VMSINSTAL. | Package management is Port collections and packages. |
| It has the preferred license of Proprietary. | Preferred license is BSD. |
| File systems supported by OpenVMS are Files-11, ISO 9660, NFS and CIFS. | File systems supported are UFS2, ZFS, ext2, ext3, FAT, ISO 9660, UDF and NFS. |