Compare commits
3 Commits
d292babfef
..
main
| Author | SHA1 | Date | |
|---|---|---|---|
| f4e9b523a8 | |||
| 6829b9d825 | |||
| 0b50094774 |
@@ -0,0 +1,3 @@
|
|||||||
|
html {
|
||||||
|
scroll-padding-top: 5rem;
|
||||||
|
}
|
||||||
+133
-3
@@ -1,9 +1,139 @@
|
|||||||
+++
|
+++
|
||||||
title = "ISA Documentation"
|
title = "ISA Documentation"
|
||||||
|
|
||||||
comments = false
|
|
||||||
breadcrumbs = false
|
breadcrumbs = false
|
||||||
toc = false
|
|
||||||
|
|
||||||
cascade.type = "docs"
|
cascade.type = "docs"
|
||||||
+++
|
+++
|
||||||
|
|
||||||
|
The best way to understand a CPU is to take one apart. The Little Computer
|
||||||
|
family was designed with exactly that in mind: every detail is intentional,
|
||||||
|
every constraint has a reason. The three canonical architectures (LC-2, LC-3,
|
||||||
|
and LC-3b) were built to be studied, not just used, and their internal structure
|
||||||
|
reflects that philosophy at every level.
|
||||||
|
|
||||||
|
At the core, all three share the same foundation. The address bus is 16 bits
|
||||||
|
wide, giving a linear address space of 65,536 locations, no segmentation, no
|
||||||
|
paging, no virtual memory. Each architecture uses eight general-purpose
|
||||||
|
registers (R0-R7), a program counter (PC), and a 3-bit condition code register
|
||||||
|
(CC) that tracks the sign of the last result written to a register: negative,
|
||||||
|
zero, or positive. Every instruction is exactly 16 bits wide, with the top 4
|
||||||
|
bits reserved for the opcode, a constraint that shapes the design of every
|
||||||
|
instruction in the set.
|
||||||
|
|
||||||
|
Each successive version refines this foundation rather than replacing it. LC-2
|
||||||
|
and LC-3 pair the 16-bit address bus with a 16-bit data bus, meaning each memory
|
||||||
|
location holds a full 16-bit word - 128 KiB of addressable memory in total.
|
||||||
|
LC-3b narrows the data bus to 8 bits: memory locations hold a single byte,
|
||||||
|
halving the addressable memory to 64 KiB and introducing byte-level memory
|
||||||
|
operations absent in the other two. LC-3 and LC-3b also add privilege levels - a
|
||||||
|
distinction between user mode and supervisor mode - which the original LC-2 does
|
||||||
|
not have.
|
||||||
|
|
||||||
|
What none of them have is equally telling: no flags register, no status
|
||||||
|
register, no variable-length instructions, no complex addressing hierarchies.
|
||||||
|
The simplicity is not a limitation, it is the point.
|
||||||
|
|
||||||
|
Beyond the canonical three, two other architectures are documented here. LC-1
|
||||||
|
predates LC-2 and represents the earliest iteration of the family: simpler, more
|
||||||
|
constrained, and rarely discussed today. LC-3.2 sits at the other end: an
|
||||||
|
unofficial extension that expands the address space to 32 bits and reduces the
|
||||||
|
word size to 8 bits, bringing the family closer to the real-world architectures
|
||||||
|
it was never meant to replicate.
|
||||||
|
|
||||||
|
|
||||||
|
|
||||||
|
## How the Family Evolved
|
||||||
|
|
||||||
|
{{% steps %}}
|
||||||
|
|
||||||
|
### LC-1
|
||||||
|
|
||||||
|
The starting point of the family. An accumulator-based architecture with a
|
||||||
|
single register, a 13-bit address space, and just 8 instructions. Extremely
|
||||||
|
minimal by design. Closer to a teaching toy than a real CPU.
|
||||||
|
|
||||||
|
### LC-2
|
||||||
|
|
||||||
|
LC-2 marks the transition from a single-accumulator model to a proper register
|
||||||
|
file. The initial revision introduced 4 registers and N,Z condition codes. This
|
||||||
|
is a first step away from the extreme minimalism of LC-1.
|
||||||
|
|
||||||
|
The final revision settled on 8 registers (R0-R7) and three condition codes (N,
|
||||||
|
Z, P), the blueprint that all subsequent canonical architectures would inherit.
|
||||||
|
The result is an architecture that feels closer to early 8-bit CPUs like the
|
||||||
|
Intel 8080: still simple, but no longer a toy.
|
||||||
|
|
||||||
|
### LC-3
|
||||||
|
|
||||||
|
The most widely known architecture in the family. The initial revision
|
||||||
|
introduced a stack that grows toward zero, PC-relative offsets using sign
|
||||||
|
extension instead of zero extension, and register-relative addressing with
|
||||||
|
signed offsets.
|
||||||
|
|
||||||
|
The revised version added privileged memory, available in two models: fixed and
|
||||||
|
selectable. LC-3 has been adopted by numerous universities as the basis for
|
||||||
|
introductory computer architecture courses.
|
||||||
|
|
||||||
|
### LC-3b
|
||||||
|
|
||||||
|
A variant of LC-3 with an 8-bit data bus instead of 16-bit, halving the
|
||||||
|
addressable memory to 64 KiB but introducing byte-level memory operations.
|
||||||
|
|
||||||
|
Working at the byte level makes LC-3b the closest of the canonical architectures
|
||||||
|
to how real-world CPUs actually handle memory.
|
||||||
|
|
||||||
|
### LC-3.2
|
||||||
|
|
||||||
|
An unofficial extension, not created by Patt and Patel, that expands the address
|
||||||
|
bus to 32 bits and narrows the data bus to 8 bits - the same combination used by
|
||||||
|
early x86 processors. It is the only architecture in the family that bridges the
|
||||||
|
gap between the LC teaching model and real-world CPU design, which is what makes
|
||||||
|
it worth studying.
|
||||||
|
|
||||||
|
{{% /steps %}}
|
||||||
|
|
||||||
|
|
||||||
|
|
||||||
|
## At a Glance
|
||||||
|
|
||||||
|
| ISA | Data Bus Width | Address Bus Width | Memory | Registers | Instruction Width | Privilege Levels | Status |
|
||||||
|
|:------:|:--------------:|:-----------------:|:-------:|:----------------------------------:|:-------------------------:|:----------------:|:-----------:|
|
||||||
|
| LC-1 | 16-bit | **13-bit** | 16 kiB | **One 16-bit accumulator** | 16-bit (**3-bit** opcode) | No | **Legacy** |
|
||||||
|
| LC-2 | 16-bit | 16-bit | 128 kiB | Eight 16-bit registers (R0-R7) | 16-bit (4-bit opcode) | No | Canonical |
|
||||||
|
| LC-3 | 16-bit | 16-bit | 128 kiB | Eight 16-bit registers (R0-R7) | 16-bit (4-bit opcode) | Yes (2) | Canonical |
|
||||||
|
| LC-3b | **8-bit** | 16-bit | 64 kiB | Eight 16-bit registers (R0-R7) | 16-bit (4-bit opcode) | Yes (2) | Canonical |
|
||||||
|
| LC-3.2 | **8-bit** | **32-bit** | 4 GiB | **Eight 32-bit registers** (R0-R7) | 16-bit (4-bit opcode) | Yes (2) | **Variant** |
|
||||||
|
|
||||||
|
|
||||||
|
|
||||||
|
## A Note on Sources
|
||||||
|
|
||||||
|
All data in this section is sourced from primary references: the original books,
|
||||||
|
course materials, and official publications by Patt and Patel. Every claim is
|
||||||
|
cited. If you spot an error or know of a better source, contributions are
|
||||||
|
[welcome via GitHub](https://github.com/little-emulator/docs/issues).
|
||||||
|
|
||||||
|
|
||||||
|
|
||||||
|
## Explore the Architectures
|
||||||
|
|
||||||
|
Each architecture in the family has its own dedicated reference page,
|
||||||
|
documenting the instruction set, registers, memory model, and encoding in full
|
||||||
|
detail.
|
||||||
|
|
||||||
|
### Core Architectures
|
||||||
|
|
||||||
|
<br>
|
||||||
|
{{< hextra/feature-grid cols="2" >}}
|
||||||
|
{{< hextra/feature-card title="LC-2" icon="chip" link="lc-2" >}}
|
||||||
|
{{< hextra/feature-card title="LC-3" icon="chip" link="lc-3" >}}
|
||||||
|
{{< hextra/feature-card title="LC-3b" icon="chip" link="lc-3b" >}}
|
||||||
|
{{< /hextra/feature-grid >}}
|
||||||
|
|
||||||
|
### Beyond the Canon
|
||||||
|
|
||||||
|
<br>
|
||||||
|
{{< hextra/feature-grid cols="2" >}}
|
||||||
|
{{< hextra/feature-card title="LC-1" icon="bookmark" link="lc-1" subtitle="**Legacy**: The accumulator-based predecessor" >}}
|
||||||
|
{{< hextra/feature-card title="LC-3.2" icon="cube" link="lc-3.2" subtitle="**Variant**: An unofficial 32-bit extension" >}}
|
||||||
|
{{< /hextra/feature-grid >}}
|
||||||
|
|||||||
@@ -0,0 +1,59 @@
|
|||||||
|
{{- /*
|
||||||
|
cite-talk - Chicago Notes style conference presentation citation shortcode.
|
||||||
|
Renders a fully formatted Chicago Notes citation for a conference talk,
|
||||||
|
keynote, poster, or similar presentation.
|
||||||
|
|
||||||
|
@param {string} author Author(s) in "Lastname, Firstname" format.
|
||||||
|
@param {string} title Presentation title (rendered in quotes).
|
||||||
|
@param {string} type Type of presentation (e.g. "Keynote", "Paper", "Poster", "PowerPoint").
|
||||||
|
@param {string} event Conference or event name.
|
||||||
|
@param {string} location City and state/country of the event.
|
||||||
|
@param {string} date Date or month/year of the event.
|
||||||
|
@param {string} url URL of the resource (required if online).
|
||||||
|
@param {string} format File format, if applicable (e.g. "PDF", "PowerPoint").
|
||||||
|
@param {string} accessed Date the resource was accessed.
|
||||||
|
@param {string} url-archived Archived version URL (e.g. Wayback Machine).
|
||||||
|
@param {string} url-archived-date Date of archival (e.g. "June 15, 2017").
|
||||||
|
|
||||||
|
@example {{< cite-talk
|
||||||
|
author="Patt, Yale N."
|
||||||
|
title="LC-3, x86, or MIPS: The First ISA for Students to Study"
|
||||||
|
type="Keynote"
|
||||||
|
event="Workshop on Computer Architecture Education"
|
||||||
|
location="San Diego, CA"
|
||||||
|
date="June 9, 2007"
|
||||||
|
url="https://www.csc2.ncsu.edu/faculty/efg/wcae/ISCA2007/FinalProgram.html"
|
||||||
|
format="PowerPoint presentation"
|
||||||
|
accessed="June 8, 2026"
|
||||||
|
url-archived="https://web.archive.org/web/20250129103001/https://www.csc2.ncsu.edu/faculty/efg/wcae/ISCA2007/FinalProgram.html"
|
||||||
|
url-archived-date="January 29, 2025"
|
||||||
|
>}}
|
||||||
|
*/ -}}
|
||||||
|
|
||||||
|
{{- $author := .Get "author" -}}
|
||||||
|
{{- $title := .Get "title" -}}
|
||||||
|
{{- $type := .Get "type" -}}
|
||||||
|
{{- $event := .Get "event" -}}
|
||||||
|
{{- $location := .Get "location" -}}
|
||||||
|
{{- $date := .Get "date" -}}
|
||||||
|
{{- $url := .Get "url" -}}
|
||||||
|
{{- $format := .Get "format" -}}
|
||||||
|
{{- $accessed := .Get "accessed" -}}
|
||||||
|
{{- $urlArchived := .Get "url-archived" -}}
|
||||||
|
{{- $urlArchivedDate := .Get "url-archived-date" -}}
|
||||||
|
|
||||||
|
{{- if $author }}{{ $author }}. {{ end -}}
|
||||||
|
{{- if $title }}"{{ $title }}"{{ end -}}
|
||||||
|
{{- if or $type $event $location $date }} (
|
||||||
|
{{- $type }}
|
||||||
|
{{- if and $type $event }} presentation at {{ end }}
|
||||||
|
{{- if and (not $type) $event }}Presentation at {{ end }}
|
||||||
|
{{- $event }}
|
||||||
|
{{- if and $event $location }}, {{ end }}
|
||||||
|
{{- $location }}
|
||||||
|
{{- if and (or $event $location) $date }}, {{ end }}
|
||||||
|
{{- $date }}).
|
||||||
|
{{- end -}}
|
||||||
|
{{- if $url }} <a href="{{ $url }}" target="_blank">{{ $url }}</a>{{ with $format }} ({{ . }}){{ end }}.{{ end -}}
|
||||||
|
{{- if $accessed }} Accessed {{ $accessed }}.{{ end -}}
|
||||||
|
{{- if $urlArchived }} (<a href="{{ $urlArchived }}" target="_blank">Archived{{ with $urlArchivedDate }} on {{ . }}{{ end }}</a>){{ end -}}
|
||||||
Reference in New Issue
Block a user