Added main LC2 page

content/isa-docs/lc-2/_index.md

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 +++
 title = "LC-2"
 weight = 100
-draft = true
 +++
+
+LC-2 is the first architecture in the Little Computer family that looks like a
+real ISA. Built as a successor to the minimalist LC-1,[^lc1-successor] it
+introduced eight general-purpose registers and a three-state condition code
+system, the foundation that every subsequent architecture in the family would
+inherit.[^lc-family-evolution]
+
+[^lc1-successor]: {{< cite-ics edition="1" chapter="1. Welcome Aboard" page="2" >}}
+[^lc-family-evolution]: {{< 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"
+>}}
+
+LC-2 operates on a 16-bit data bus and a 16-bit address bus, giving a linear
+address space of 65,536 word-sized locations, 128 KiB of addressable memory in
+total. Every instruction is exactly 16 bits wide, with the top 4 bits reserved
+for the opcode. This fixed encoding means that the opcode, operands, and any
+immediate values must all fit within those 16 bits, a constraint that shapes the
+design of every instruction in the set.[^lc2-overview]
+
+[^lc2-overview]: {{< cite-ics edition="1" chapter="Appendix A: The LC-2 ISA" page="429" >}}
+
+## Registers
+
+LC-2 has a small but complete set of registers. All registers are 16 bits wide,
+matching the data bus width of the architecture.
+
+### General-Purpose Registers
+
+LC-2 provides eight general-purpose registers, named R0 through R7. They are
+symmetric: no register has a special hardware role, and any of them can be used
+as a source or destination in any instruction that operates on
+registers.[^lc2-overview] That said, two registers have a conventional role: R6
+is typically used as the stack pointer,[^lc2-stack] [^lc2-rti] and R7 is used
+by some instructions to store the return address.[^lc2-jsr-jsrr] [^lc2-ret]
+[^lc2-trap] See the [instructions page](instructions/) for details.
+
+[^lc2-stack]: {{< cite-ics edition="1" chapter="10.1.3 Implementation in Memory" page="197" page-end="200" >}}
+[^lc2-rti]: {{< cite-ics edition="1" chapter="Appendix A.3 The Instruction Set" page="444" >}}
+[^lc2-jsr-jsrr]: {{< cite-ics edition="1" chapter="Appendix A.3 The Instruction Set" page="436" page-end="437" >}}
+[^lc2-ret]: {{< cite-ics edition="1" chapter="Appendix A.3 The Instruction Set" page="443" >}}
+[^lc2-trap]: {{< cite-ics edition="1" chapter="Appendix A.3 The Instruction Set" page="448" >}}
+
+### Special-Purpose Registers
+
+Beyond the general-purpose registers, LC-2 has some special-purpose registers
+that control the execution of the processor. None of these registers are
+directly accessible from assembly, with the exception of the CC, which is
+implicitly read by conditional branch instructions.
+
+* The **Program Counter** (**PC**) holds the address of the next instruction to
+  be fetched from memory. It is incremented by 1 after each fetch, before the
+  instruction is executed, so that by the time the instruction runs, the PC
+  already points to the following one.[^lc2-fetch]
+
+  [^lc2-fetch]: {{< cite-ics edition="1" chapter="4.2.2 The Instruction Cycle" page="82" page-end="83" >}}
+
+* The **Instruction Register** (**IR**) holds the instruction currently being
+  executed. After the PC is used to fetch an instruction from memory, the
+  instruction is loaded into the IR, where it remains for the duration of the
+  decode and execute phases.[^lc2-fetch]
+
+* The **Memory Address Register** (**MAR**) holds the address of the memory
+  location to be accessed. Before any memory operation, the address is loaded
+  into the MAR, which then drives the address bus during the read or write
+  cycle.[^lc2-memory]
+
+  [^lc2-memory]: {{< cite-ics edition="1" chapter="4.1.1 Memory" page="75" page-end="77" >}}
+
+* The **Memory Data Register** (**MDR**) holds the data being transferred to or
+  from memory. On a read, the MDR receives the value fetched from the location
+  addressed by the MAR. On a write, the MDR holds the value to be stored before
+  it is placed onto the data bus.[^lc2-memory]
+
+* The **Condition Code** register (**CC**) is a 3-bit register that tracks the
+  sign of the last value written to any general-purpose register. It has three
+  mutually exclusive states: N (negative), Z (zero), and P (positive). Exactly
+  one of the three bits is set at any given time.[^lc2-overview]
+
+  Not all instructions update the CC. Only instructions that write a value to a
+  general-purpose register will modify it. As an example, an `ADD` instruction
+  that stores its result in R0 will update the CC based on the sign of that
+  result, while a `STR` instruction that writes to memory will
+  not.[^lc2-condition-codes]
+
+  [^lc2-condition-codes]: {{< cite-ics edition="1" chapter="5.1.7 Condition Codes" page="95" >}}
+
+## Interrupt Support
+
+LC-2 provides basic support for interrupts via the `RTI` (Return from
+Interrupt) instruction. When an interrupt is serviced, the processor saves the
+current status onto the stack and jumps to the interrupt handler. `RTI`
+restores the PC from the stack when the handler returns. The ISA does not
+specify the interrupt protocol, the bus signaling, or the acknowledgment
+mechanism. These details are left to the implementation.

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+<sup>[<i><a href="https://en.wikipedia.org/wiki/Citation_needed" target="_blank">citation needed</a></i>]</sup>