computer science

System Service Descriptor Table

System Service Descriptor Table is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand System Service Descriptor Table rather than just read about it. In short: The System Service Descriptor Table (SSDT) is an internal dispatch table within Microsoft Windows. Function The SSDT maps syscalls to kernel function addresses.

Key takeaways

System Service Descriptor Table belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect System Service Descriptor Table to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of System Service Descriptor Table from memory before moving on to harder problems.

Reference excerpt

The System Service Descriptor Table (SSDT) is an internal dispatch table within Microsoft Windows.

Function The SSDT maps syscalls to kernel function addresses. When a syscall is issued by a user space application, it contains the service index as parameter to indicate which syscall is called. The SSDT is then used to resolve the address of the corresponding function within ntoskrnl.exe. In modern Windows kernels, two SSDTs are used: One for generic routines (KeServiceDescriptorTable) and a second (KeServiceDescriptorTableShadow) for graphical routines. A parameter passed by the calling userspace application determines which SSDT shall be used.

Hooking Modification of the SSDT allows to redirect syscalls to routines outside the kernel. These routines can be either used to hide the presence of software or to act as a backdoor to allow attackers permanent code execution with kernel privileges. For both reasons, hooking SSDT calls is often used as a technique in both Windows kernel mode rootkits and antivirus software. In 2010, many computer security products which relied on hooking SSDT calls were shown to be vulnerable to exploits using race conditions to attack the products' security checks.

Worked examples

Example 1 — a first encounter with System Service Descriptor Table

Start with the simplest possible case. Write down what System Service Descriptor Table claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to System Service Descriptor Table before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about System Service Descriptor Table ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of System Service Descriptor Table

In research: System Service Descriptor Table appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses System Service Descriptor Table in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: System Service Descriptor Table is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer security, Microsoft Windows stubs, Windows NT kernel, so understanding it makes those chapters shorter.
In everyday life: Look for System Service Descriptor Table outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study System Service Descriptor Table in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what System Service Descriptor Table means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain System Service Descriptor Table out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is System Service Descriptor Table in simple terms?

The System Service Descriptor Table (SSDT) is an internal dispatch table within Microsoft Windows. Function The SSDT maps syscalls to kernel function addresses.

Why does System Service Descriptor Table matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study System Service Descriptor Table?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on System Service Descriptor Table.