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The PDP-8 predates the modern notion of an 8-bit byte. Back in the PDP-8's day, a "byte" was a more slippery concept. You could speak of 6-bit bytes, 7-bit bytes, 9-bit bytes... It all depended on what your particular task needed.²
Since the PDP-8 uses a 12-bit native word size, 6-bit bytes are quite common in the PDP-8 world, often used for some kind of "packed [ASCII](https://en.wikipedia.org/wiki/ASCII)" representation. [One common scheme](http://homepage.cs.uiowa.edu/~jones/pdp8/faqs/#charsets) gets rid of most of the 32 control characters defined in 7-bit ASCII, all of the lowercase letters, and a whole bunch of the punctuation in order to pack two characters into a 12-bit PDP-8 word. There are actually a few different 6-bit packed ASCII representations for the PDP-8, so you have to know which scheme you're looking at before you can turn the data back into 7-bit ASCII.
The PDP-8 was being designed at about the same time as the first versions of ASCII,³ as well as around the same time as the first wildly popular ASCII terminal, the [Teletype Model 33](https://en.wikipedia.org/wiki/Teletype_Model_33).⁴
When dealing with such terminals and the included paper tape reader, PDP-8s generally deal in either 7-bit or 8-bit bytes. When we're talking about 8-bit bytes, we aren't talking about the "[high-ASCII](https://en.wikipedia.org/wiki/Extended_ASCII)" stuff that infested the PC world in the late 1970s and 1980s before [Unicode](https://en.wikipedia.org/wiki/Unicode) was invented. When a PDP-8 reads in plain ASCII text from a terminal as 8-bit bytes, the eighth bit is a [parity bit](https://en.wikipedia.org/wiki/Parity_bit), meant to detect read errors only.
When dealing with such terminals and the included paper tape reader, PDP-8s generally deal in either 7-bit or 8-bit bytes. When we're talking about 8-bit bytes, we aren't talking about the "[high-ASCII](https://en.wikipedia.org/wiki/Extended_ASCII)" stuff that infested the PC world in the late 1970s and 1980s before [Unicode](https://en.wikipedia.org/wiki/Unicode) was invented.
Much existing PDP-8 software that reads in plain ASCII text from a terminal as 8-bit bytes treats the eighth bit on standard DEC format paper tapes as a [mark parity bit](https://en.wikipedia.org/wiki/Parity_bit), rather than as an error-detecting parity bit or as a data bit.
Full 8-bit reads from the terminal did still commonly occur on PDP-8s though. The most common schemes are the [RIM loader](https://www.pdp8online.com/pdp8cgi/query_docs/tifftopdf.pl/pdp8docs/dec-08-lraa-d.pdf) and [BIN loader](http://www.pdp8online.com/pdp8cgi/query_docs/tifftopdf.pl/pdp8docs/dec-08-lbaa-d.pdf) binary paper tape formats. See those PDFs for details, but for our purposes here, it's only important to note that both schemes expressed two 12-bit PDP-8 words as three 8-bit bytes, one per row on the paper tape when punching it, and thus read back into the machine one 8-bit byte at a time. A large part of the machine code in the RIM and BIN loaders is concerned with rearranging these 8-bit bytes into 12-bit PDP-8 words.
# Words
We speak of PDP-8 memory in terms of words, rather than bytes, because the smallest addressable unit of memory is the word, and the modern 8-bit byte doesn't divide evenly into the PDP-8 word size. That is, you can't retrieve just one byte of memory, so it is more useful to think about it in words.
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