CISC (Complex Instruction Set Computer)

<aside> 💡 **Characteristics of CISC

• Variable Instruction Length** (length of instruction varies based on what was needed)
• More powerful instructions (such as push, pop, scale-factor addressing versus in RISC we had to change the stack pointer and use sw/lw)

Having more powerful instructions also keeps the program short (1 instruction in CISC versus 4 in RISC)

Downsides of CISC

• Incremental ISA (backwards-compatible, supporting old instructions that aren’t used anymore)

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<aside> 💡 Why was CISC dominant in the early days of computers? How CISC architectures tended to keep programs short?

CISC was dominant in the early days of computing because during that time there was a low amount of secondary storage meaning that it was important to keep program sizes small.

CISC architectures tend to keep programs short due to the fact that they have variable length instructions, meaning that the size of an instruction can vary.

For instance, in the fake Arhin Architecture, the nop is only 4 bits (1011) whereas an instruction like add $18, var in x64 might take up more bytes.

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RISC (Reduced Instruction Set Computer)

<aside> 💡 **Characteristics of RISC

• Fixed Instruction Length** (an instruction will always be 32 bits/4 bytes, even if it doesn’t need 4 bytes)

Fixed Instruction Length is also a benefit in the Instruction Cycle as you don’t need to decode to know where the next instruction is

• Better Pipelining Support
• Load/Store Architecture

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Load Store Architecture

<aside> 💡 ****(i.e. load variable’s value from memory, update the value, then store it)

What makes an ISA a load-store architecture? What makes an ISA a load-store architecture is that there are dedicated instruction that can loads values from memory + another instruction that stores/writes values to memory. There is no instruction in a load-store architecture that does both.

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