【Semiconductor】18. What Was PSRAM Intended to Achieve?

— The Premise of Reusing DRAM

topics: [“Semiconductor”, “PSRAM”, “DRAM”, “Memory”, “Architecture”]

🧭 Introduction — Why Did PSRAM Become an Option?

Around the year 2000,
a memory called PSRAM (Pseudo-SRAM) was introduced to the market.

Its external interface looked like SRAM,
while DRAM cells were used internally.

In this article, we organize:

Discussions of failures and limitations are deferred to the next article.

📱 Background of PSRAM — Constraints of Mobile Systems

In the late 1990s,
systems for mobile devices faced the following requirements simultaneously:

However, the available options at the time were polarized.

Type Characteristics
SRAM High speed, simple control, high cost
DRAM High density, low cost, complex control

PSRAM was conceived to fill
the gap between usability and cost.

🎯 The Core Concept of PSRAM

The central idea of PSRAM was extremely simple.

Keep the inside as DRAM,
but make it look like SRAM from the outside

Rather than abandoning DRAM and creating a new type of cell,
the premise was to maximize reuse of existing DRAM technology.

🏗 Key Points of the Internal Structure

① Using the DRAM Cell Array as Is

There is no special physical design
unique to PSRAM at the cell level.

② Internal Refresh Control

The defining feature of PSRAM is that it:

As a result,
it can be accessed externally with the same ease as SRAM.

③ An Access-Priority Control Policy

The internal control policy was:

This choice was reasonable for mobile applications because it:

🧩 Assumed Usage Conditions

PSRAM was designed as a
memory dedicated to mobile applications.

The assumed usage conditions were:

These differed clearly
from the conventional usage patterns of DRAM.

⚖️ The Decisive Difference Between DRAM and PSRAM

The critical point here is the following:

PSRAM uses DRAM cells as-is,
but it is not used in the same way as DRAM

This difference would later take on major significance.

✅ A “Rational Design” for Its Time

The structure of PSRAM was:

In this sense,
it was an extremely rational design for its time.

At this stage, words such as:

had not yet become reality.

🧾 Summary (Structure)

PSRAM was a memory with a
clear and consistent objective:

The problem was that
this usage model quietly violated
the physical assumptions of DRAM.

🔗 Primary Sources (References)

⏭ Next

In the next article,
we will cover the failures that actually surfaced in PSRAM.

Why Pause and Disturb
became problematic in combination.

No evaluation, no hindsight—
only the facts.