Computer Architecture
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In a nutshell: Let's recap the last four modules: we've built some elementary logic gates (module 1), and then used them to build an ALU (module 2) and a RAM (module 3). We then played with low-level programming (module 4), assuming that the overall computer is actually available. In this module we assemble all these building blocks into a general-purpose 16-bit computer called Hack. We will start by building the Hack Central Processing Unit (CPU), and we will then integrate the CPU with the RAM, creating a full-blown computer system capable of executing programs written in the Hack machine language.
Key concepts: Von Neumann and Harvard architectures, the stored program concept, fetch-execute cycle, data bus, instruction bus, CPU, computer design.
The ALU loads information from the Data bus and manipulates it using the Control bits.
Both Data and Addresses are stored in the registers.
Put the location of the next instruction into the "address" of the program memory
Get the instruction code itself by reading the memory contents at that location
The instruction code specifies "what to do"
Which arithmetic or logical instruction
What memory to access (read/write)
if/where to jump
...
Often, different subsets of the bits control different aspects of the operation
Executing the operation involves also accessing registers and/or data memory
The fetch part of the cycle reads from the program memory, and the execute part reads from data memory.
Variant of von Neumann
Keeps Program and Data in two separate memory modules
Complication avoided
The Hack CPU Abstraction
a 16-bit processor designed to:
Execute the current instruction
Figure out which instruction to execute next (written in the Hack language)
Abstraction: A computer capable of running programs written in the Hack machine language.
RAM: 16-bit / 16K RAM chip
Screen: 16-bit / 8K memory chip with a raster display side-effect
Keyboard: 16-bit register with a keyboard side-effect
probe memory register 24576
Hardware implementation: plug-and-play ROM chips
Hardware simulation: programs are stored in text files; program loading is emulated by the built-in ROM chip
