On-Chip Memory Beyond Cache: Understanding Registers and Explicit Addressing

When discussing CPU architecture, understanding the various levels of on-chip memory, including cache and registers, is crucial. This article will explore these components in depth, detailing how they differ and how they are addressed in different types of chips.

Registers: A Fundamental Part of On-Chip Memory

Registers are the most basic and essential elements of on-chip memory within most CPU architectures. They are closely tied to CPU instructions, which means they can both read from and write to these registers. However, in most consumer systems, whether it’s a laptop, desktop, or even a simple microcontroller, the primary on-chip memory components are the cache and the registers.

Cache on Consumer Systems

In consumer systems such as laptops and desktops, the cache and registers are typically the only on-chip memory components. Cache hierarchies can include L1, L2, and even L3 cache. Some high-end chips even feature an L3 cache. Additionally, there may be a write-pipe for dealing with pending writes. However, only the registers are directly accessible on these chips.

Microcontrollers and Other Chips

Microcontrollers, on the other hand, have a different architecture where you find registers, flash memory to hold the program, and RAM for usual operations. Persistent data might be stored in EEPROM. The specifics of what chips include and how they are addressed can vary widely among vendors and models.

For instance, in the ESP32 family, the flash program memory can be addressed by providing its address in an instruction. In the AVR family of microcontrollers, you must specify the address in a special register and then use a special instruction. Accessing flash is relatively slow compared to other memory types.

EEPROM usage in microcontrollers often comes with strict timing and power penalties, as do synchronous flash writes. Most systems that require persistent data rely on external EEPROM or FRAM chips, accessed via I2C, which can significantly slow down the process.

Cache in Modern CPUs

The design of cache in modern CPUs has evolved significantly. Early RISC architectures, for instance, placed L1 caches (both instruction and data) as separate entities, often with an off-chip L2 cache. Today, L2 caches are typically on-chip per core, with multi-core configurations featuring a shared L3 cache. All levels of cache are addressable, either directly or indirectly, as they form subsets of the overall memory space.

Explicit Addressing of On-Chip Memories

Explicit addressing is a key aspect of interacting with on-chip memory. This is particularly true for microcontrollers where the programmer must explicitly specify the address in the instruction or special register.

For example:

In the ESP32 family, you can access flash program memory directly by providing an address in an instruction. In AVR family microcontrollers, you must load the address into a special register and then use a special instruction. Access to flash and EEPROM comes with timing and power penalties.

Understanding the explicit addressing mechanisms for different types of chips is crucial, especially when optimizing code for performance or ensuring efficient use of the available memory.

Conclusion

On-chip memory, from registers to various cache levels, plays a vital role in CPU and microcontroller design. Explicit addressing is a critical feature that differentiates chips, particularly in microcontrollers like the ESP32 and AVR families. Properly understanding and utilizing these components can significantly impact the performance and efficiency of a device.