This line of operating systems started out life as a series of patches to AT&T UNIX which was introduced to the University of Berkeley by Ken Thompson whilst on sabbatical in 1977. From the 1BSD TAPE file included in the CSRG archive CD set:
Berkeley UNIX Software Tape Jan 16, 1978 TP 800BPI
The first release came with things such as the ex editor, ashell and Pascal compiler as an add-on for UNIX v7, running on a PDP-11. Over the life time of the CSRG they produced releases which included vi, csh, the IPv4 TCP/IP network stack, the virtual memory subsystem (the kernel being named vmunix, parodied by Linux as vmlinuz) and UFS.
The distribution tapes were only available to AT&T licensees; over time the code base of the distribution grew increasingly independent from AT&T UNIX. At the same time the cost of the AT&T license continued to increase as well. Starting out at a cost of $10000 and reaching north of $250000 in the late 80's. According to Kirk McKusick there was pressure to release the independently developed components of the CSRG so the community could benefit from the use of things such as the network stack without purchasing a costly license. This resulted in several release, comprised mostly of the code developed outside of AT&T such as 4.3BSD-Net/1, Net/2, 4.4BSD-Lite & Lite2. "Mostly" in that with the release of Net/2 AT&T file a lawsuit against the University of California for alleged code copying and theft of trade secrets.
During its lifetime, BSD saw itself being run on several CPU architectures from the DEC PDP-11, VAX to the MIPS, HP 9000 and Motorola 68000 to name a few. These ports along with the Power 6/32 helped to improve the portability of the code base. The code base was deemed to be 90% platform independent, the remaining 10% being mostly related to the VM subsystem which was platform specific. As with AT&T UNIX, portability & migration between different systems was part of the nature of the code base, from the beginning.
The 4.3BSD-Net/2 code base was used as the basis for a port to the Intel 386, resulting in 386BSD (free) & BSD386 (commercial) releases.