Fairchild 6800 - Page 1
Forum section: electronics / general

author: westfw
author_rank: Super Contributor
timestamp: September 17, 2020, 06:55:03 am
content: Look what I found in some boxes of old documents!

author: David Hess
author_rank: Super Contributor
timestamp: September 18, 2020, 11:48:52 am
content: I wonder how a 16-bit address bus results in 65K of address space, while not resulting in 66K of address space if you round up.

author: coppice
author_rank: Super Contributor
timestamp: September 18, 2020, 11:57:51 am
content: I'd forgotten Fairchild ever made 6800 parts. Fairchild was one of the more successful microprocessor vendors at that time, with their own F3870 parts.

author: Bruce Abbott
author_rank: Frequent Contributor
timestamp: September 20, 2020, 05:24:51 am
content: Quote from: David Hess on September 18, 2020, 11:48:52 amCustomers might get upset about not getting the advertised amount of RAM. By quoting 65k they guarantee that at least 65000 Bytes are possible.I still have the MC6802 from the computer I designed and built in 1981. It had a MC6847 video display generator and 8k of battery-backed CMOS RAM using HM6116's. The RAM was shared between the CPU and VDG using both halves of the E clock. The whole thing was not much larger than the keyboard, with a very compact circuit board made from Veroboard and wired up with aVerowire wiring pen.The 6800 is slow and inefficient compared to later 8 bit CPUs such as the 6502 and Z80, but it was much faster than the CDP1802 in my first computer, a HUG1802 (precursor of theETI-660). I made an EPROM programmer for the HUG1802 and used it to burn the system ROM for the 6802. Back in those days I didn't have access to any firmware, so all the code was written from scratch. Of course I didn't have an assembler either, so I wrote programs down on paper and translated them into hex codes for entering into the computer. Fun times!I am thinking of reproducing my 6800 design using the original CPU, but with BASIC in ROM and a floppy drive (stuff I couldn't get or afford back then) and/or an SD Card slot or USB port.

author: coppice
author_rank: Super Contributor
timestamp: September 20, 2020, 11:20:01 am
content: Quote from: David Hess on September 18, 2020, 11:48:52 amIn the 1970s people were not used to k == 1000 and K == 1024 as they are today. Many datasheets referred to a 65k address space for a 16 bit bus, or even a 65K address space. The capital K had not yet picked up its meaning as 1024. Lots of engineering notes capitalised everything. This could often confuse milli-something with mega-something, in ways that had to be resolved by context.

author: David Hess
author_rank: Super Contributor
timestamp: September 20, 2020, 11:00:41 pm
content: Quote from: coppice on September 20, 2020, 11:20:01 amI do not remember that being the case, but maybe Fairchild thought that way. Not much later we *did* have things like 63K systems when the monitor ROM took the remaining 1K.Are there any other examples? I have a Signetics book here but they were not thinking in terms of maximum address space.

author: Bruce Abbott
author_rank: Frequent Contributor
timestamp: September 21, 2020, 08:50:03 am
content: Apparently describing a 65536 Byte address space as '65k' was quite popular at the time. I found several CPU datasheets that did it, including the Synertek SY6500, California Micro Devices G65SC02, MOS Technology MCS6501, and Hitachi HD6800.Also inPopular Electronics Nov 1975edition is an article comparing the Altair 680 (6800 CPU) to the Altair 8800 (8080 CPU), which describes both systems as having a maximum memory size of '65k bytes'.