What is the difference between ABI and ISA?

The ISA is the set of instructions a CPU understands; the ABI is the broader binary contract — the ISA plus calling conventions, stack layout, and data alignment.

* The ISA is the inner hardware contract that the ABI wraps with calling convention, stack layout, alignment, and system-call rules. *

What ISA specifies:

• Available instructions (add, subtract, load, store, jump...)
• Registers and their purposes
• Instruction encoding (binary format)
• Memory addressing modes

What ABI adds on top:

• Calling convention: Which registers hold function arguments? (rdi, rsi, rdx... on Linux x86-64)
• Stack layout: How is the stack organized? Which way does it grow?
• Data alignment: How are structs laid out in memory?
• System calls: How do you ask the OS to do something?

Why this matters:

• Same ISA (x86-64), different ABIs → Linux binaries won't run on Windows!
• Compilers must know the ABI to generate compatible code
• Reverse engineers need to know the ABI to understand function calls

Example: sum += A[1 + i] might compile to add 4(%rdi, %rsi, 2), %rax - the ABI determined that A is in rdi and i is in rsi.

Go deeper:

• Application binary interface (Wikipedia) — the ABI layer (calling conventions, register/stack use, object formats) that sits above the raw ISA.
• Instruction set architecture (Wikipedia) — what the hardware instruction set defines, versus what the ABI adds on top.
• x86 calling conventions (Wikipedia) — how real x86/x86-64 conventions pass args, return values and clean the stack.