gem5.org
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gem5.org is the home of the gem5 simulator, an open-source, modular platform for computer-system architecture research and education. It provides customizable full-system and subsystem simulators, along with documentation, tutorials, and guides to model CPUs, memory systems, interconnects, and peripherals.
More resources on Computer Architecture
nand2tetris.org
Nand2Tetris.org offers a hands-on computer-architecture course that guides you from basic NAND gates to a working Hack computer and software stack. It provides project descriptions, HDL files, simulators, and tools (assembler, VM translator, compiler) plus links to the accompanying textbook.
cs61c.org
UC Berkeley's CS61C: Machine Structures course site, offering lecture videos, slides, labs, problem sets, and exams that teach core computer architecture concepts like CPU design, memory hierarchy, and pipelining.
riscv.org
RISC-V.org is the official hub for the RISC-V open instruction set architecture, offering architecture specifications, tooling, reference implementations, compliance resources, and community news and events for developers, educators, and organizations.
Nand2Tetris Part II
Project-based Coursera course from the Hebrew University in which learners build a stack-based virtual machine translator, a compiler for the Java-like Jack language and a small operating system for the Hack computer. Afterwards they understand how high-level code becomes executable machine instructions.
NandGame
A browser puzzle that starts with a single NAND gate and has you compose latches, adders, an ALU, memory, and finally a working CPU with its own assembler and instruction set.
Computation Structures (MIT 6.004)
Digital systems from the bottom up: logic gates and sequential circuits, instruction set design, assembly language, procedures and stacks, processor pipelining, virtual memory and concurrency. Offers 173 short lecture videos and problem sets with solutions, so learners can build and reason about a working processor.