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# Bytes
The base PDP-8 memory configuration is 4 kW of [core memory](https://en.wikipedia.org/wiki/Magnetic-core_memory). We speak of PDP-8 memory in terms of words, rather than bytes, because 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 really popular ASCII terminal, the Teletype Model 33.⁴ Thus, the PDP-8 was one of the first popular ASCII-aware machines in the world.
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.
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
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**Footnotes and Digressions**
1. The PDP-8 does have one programmer-accessible register that isn't an even multiple of 3 bits, the single-bit "link" register, but we won't be talking more about that register in this article. The term "link" in the article does not refer to this register.
2. Thus the term [octet](https://en.wikipedia.org/wiki/Octet_(computing\)), which unambiguously refers to a group of 8 bits.
3. Yes, "versions," plural. There were actually 3 major [versions of ASCII](https://en.wikipedia.org/wiki/ASCII#History), published in 1963, 1964, and 1967. Since the original PDP-8 came out in 1965 and the PDP-8/I model came out in 1968, you can see that the development of these computers coincided quite nicely with the development of ASCII itself. Thus, the PDP-8/I is one of the first machines that aware from inception of what we now think of as 7-bit ASCII.
3. Yes, "versions," plural. There were actually 3 major [versions of ASCII](https://en.wikipedia.org/wiki/ASCII#History), published in 1963, 1964, and 1967. Since the original PDP-8 came out in 1965 and the PDP-8/I model came out in 1968, you can see that the development of these computers coincided quite nicely with the development of ASCII itself. Thus, the PDP-8/I is one of the first machines that was aware from inception of what we now think of as 7-bit ASCII.
4. The most common terminal used with PDP-8s and other machines of its era was the [Teletype Model 33](https://en.wikipedia.org/wiki/Teletype_Model_33) ASR, the latter referring to the Asynchronous Send and Receive version. Although you will commonly see this referred to as an ASR-33, that is not its [proper](https://en.wikipedia.org/wiki/Teletype_Model_33#Model_33_ASR_vis-.C3.A0-vis_ASR-33) name.
4. The most common terminal used with PDP-8s and other machines of its era was the Teletype Model 33 ASR, the latter referring to the Asynchronous Send and Receive version. Although you will commonly see this referred to as an ASR-33, that is not its [proper](https://en.wikipedia.org/wiki/Teletype_Model_33#Model_33_ASR_vis-.C3.A0-vis_ASR-33) name.
As for my "wildly popular" characterization, they sold over half a million of them by the mid 1970s. That qualifies as "wildly popular" by the standards of the day. Such success wouldn't be exceeded by a computer product until the first consumer-friendly microcomputers came out.
5. Remember the notation: 4096 12-bit words is 6144 bytes, but we always speak of core memory in terms of words, not bytes. It's "4k" or "4 kW", not "6 kB".
6. Pro tip: every time the third digit of an octal address goes up by 2, it is referring to a different PDP-8 page: page 1 begins at address 200₈, page 2 begins at address 400₈, etc.
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