TLDR: An abstract machine is a theoretical model used in computer science to understand how computer systems work. It's like a mathematical function that takes inputs and produces outputs based on predefined rules. Abstract machines are different from actual machines because they focus on the step-by-step execution of programs and ignore many aspects of real hardware. They are used for theoretical analysis and as models for real-world computer systems.

An abstract machine is a way to analyze how a computer system functions. It allows computer scientists to study the behavior of programs and understand how they interact with the hardware. Abstract machines are like mathematical functions because they take inputs and produce outputs based on predefined rules. They can be used to simulate the execution of programs and analyze their complexity.

There are two types of abstract machines: deterministic and non-deterministic. A deterministic abstract machine always produces the same outputs for the same inputs, while a non-deterministic abstract machine can produce different outputs for the same input. Non-deterministic machines are useful when it's difficult or costly to find a precise solution using a deterministic approach.

One of the most well-known abstract machines is the Turing machine, which is used to study the concept of computability. Turing machines operate on a tape of symbols and can modify the symbols and move the machine's pointer. They can be either deterministic or non-deterministic.

Abstract machines can be implemented in different ways. They can be implemented in hardware using physical devices like memory and logic circuits. They can also be implemented in software or firmware, which allows for more flexibility and easier changes to the machine's behavior.

Abstract machines are used in programming language implementation. They provide a bridge between the high-level constructs of a programming language and the low-level operations of an actual machine. They define the data structures and algorithms needed to store and run programs written in a specific programming language.

Different types of programming languages have their own abstract machines. Imperative languages like Algol and Forth have their own abstract machines, as do object-oriented languages like Smalltalk and Java. String processing languages and functional programming languages also have their own abstract machines.

Abstract machines have a structure that includes a memory and an interpreter. The memory is used to store data and programs, while the interpreter executes the instructions included in the programs. The interpreter carries out operations unique to the language it is interpreting, but there are common categories of operations shared by all interpreters, such as processing primitive data, controlling the execution sequence, controlling data transfers, and managing memory.

Abstract machines can be organized into hierarchies, where each machine builds on the functionality of the level below it. At the most basic level, there is the hardware computer. Above that, there can be microprogrammed machines, operating system abstract machines, and intermediary machines like virtual machines. The hierarchy can continue with additional levels for specific applications or services.

In summary, an abstract machine is a theoretical model used in computer science to understand how computer systems function. It allows for the analysis of programs and the study of computational complexity. Abstract machines can be implemented in different ways and are used in programming language implementation. They have a structure that includes a memory and an interpreter, and they can be organized into hierarchies.