PiDP-8/I Software

# Notation

We must first settle on a bit of notation.

PDP-8 addresses are traditionally written as [octal](https://en.wikipedia.org/wiki/Octal) numbers. That means when we write 123 in this context, we aren't saying "one hundred and twenty-three," we're talking about the quantity (1 × 8²) + (2 × 8¹) + (3 × 8⁰) = 83 in ordinary decimal notation. When I write an octal number below, I will write it as 123₈ to make this clear, the subscript meaning "base 8," also called "octal." If you see a multi-digit number without the base-8 subscript, it's a decimal number.

We use octal when talking about PDP-8 addresses and memory values because the [PDP-8's major registers](http://homepage.cs.uiowa.edu/~jones/pdp8/man/registers.html) are all multiples of 3 bits in size.¹ Since an octal digit encodes as 3 [bits](https://en.wikipedia.org/wiki/Bit), that makes octal the most convenient way to write PDP-8 addresses. The more common ways to write computer numbers are inconvenient: [binary](https://en.wikipedia.org/wiki/Binary_number) takes too many digits even with the tiny PDP-8 memories, and [hexadecimal](https://en.wikipedia.org/wiki/Hexadecimal) numbers don't divide evenly by 3-bit chunks until you get to 24-bit addresses, which is beyond the PDP-8's limits.
We use octal when talking about PDP-8 addresses and memory values because the [PDP-8's major registers](http://homepage.cs.uiowa.edu/~jones/pdp8/man/registers.html) are all multiples of 3 bits in size,¹ and the PDP-8's [switch register](https://raymii.org/s/articles/Toggling_in_a_simple_program_on_the_DEC_PDP-8_and_PiDP-8_using_the_switch_register.html) is divided into groups of 3 switches. Since an octal digit encodes as 3 [bits](https://en.wikipedia.org/wiki/Bit), that makes octal the most convenient way to write PDP-8 addresses. The more common ways to write computer numbers are inconvenient: [binary](https://en.wikipedia.org/wiki/Binary_number) takes too many digits even with the tiny PDP-8 memories, and [hexadecimal](https://en.wikipedia.org/wiki/Hexadecimal) numbers encode as 4 bits each; plus, the [least common multiple](http://www.mathsisfun.com/least-common-multiple.html) of 3 and 4 is 12, meaning we could encode a full 12-bit PDP-8 word as 3 hex digits, but then we'd have no way to talk about the smaller 3-bit subdivisions, which are also useful.

Another bit of notation unique to this article, but which I hope will spread, is the unit **kiW**, meaning 1024 words of PDP-8 memory. The unit is named by analogy to [kibibytes](https://en.wikipedia.org/wiki/Kibibyte), abbreviated **kiB**. This new unit does two things for us:

1.  It distinguishes between the proper [SI](https://en.wikipedia.org/wiki/International_System_of_Units) definition of **k** and the "computer memory" meaning of **k**: 1000 vs 1024. DEC manuals will often just use **k**, and you're expected to understand that it means 1024 words, not kibibytes or kilobytes.

2.  It avoids ambiguity with **kW** meaning a kilowatt. That is not an unlikely confusion, since a fairly small PDP-8/I setup dissipates about one kilowatt of power, and a large PDP-8 setup could dissipate multiple kilowatts.
2.  It avoids ambiguity with **kW** meaning a [kilowatt](https://en.wikipedia.org/wiki/Watt#Kilowatt). That is not an unlikely confusion, since a fairly small PDP-8/I setup dissipates about one kilowatt of power, and a large PDP-8 setup could dissipate multiple kilowatts.


# Bytes

The base PDP-8 memory configuration is 4 kiW 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
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.
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.

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.

The PDP-8 has a 12-bit native word size. That is the smallest chunk of data you can address in a single instruction, and it is also the size of PDP-8 machine instructions.

Every PDP-8 instruction is a single 12-bit word, and data are stored in 12-bit core memory locations. All of the PDP-8 registers are 12 bits or smaller.

12 bits lets you address 2¹² = 4096 memory locations, which is why the basic core memory size on a PDP-8 is 4 kiW.
12 bits lets you address 2¹² = 4096 memory locations, which is why the basic [core memory](https://en.wikipedia.org/wiki/Magnetic-core_memory) size on a PDP-8 is 4 kiW.


# The 3-Level Memory Addressing System

You may be aware that the PDP-8 can be expanded to 32 kiW of memory. How does that square with all of the above?

In some CPU types, instructions are variable-width, so that an instruction which takes two operands is longer than one that takes a single operand, and a self-contained instruction is shorter still, but in the PDP-8, every instruction takes a single 12-bit word. How can a PDP-8 refer to a 12-bit address when the single-word instructions are 12 bits themselves? And how do we get beyond that to 32 kiW, which would apparently require a 15-bit address? (2¹⁵ words = 32 kiW.)