Microprocessor Design/Register File
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The register file is the component that contains all the general purpose registers of the microprocessor. The register file may not contain some of the reserved registers, such as the PC, the status register, or other special registers.
[edit] Register File
A simple register file is a set of registers and a decoder. The register file requires an address and a data input.
However, this simple register file isn't useful in a modern processor design, because there are some occasions when we don't want to write a new value to a register. Also, we typically want to read two values at once and write one value back in a single cycle. Consider the following equation:
- C = A + B
To perform this operation, we want to read two values from the register file, A and B. We also have one result that we want to write back to the register file when the operation has completed. For cases where we do not want to write any value to the register file, we add a control signal called Read/Write. When the control signal is high, the data is written to a register, and when the control signal is low, no new values are written.
In this case, it is likely advantageous for us to specify a third address port for the write address:
[edit] Register Bank
Consider a situation where the machine word is very small, and therefore the available address space for registers is very limited. If we have a machine word that can only accommodate 2 bits of register address, we can only address 4 registers. However, register files are small to implement, so we have enough space for 32 registers. The solution to this dilemma is to utilize a register bank which consists of a series of register files combined together.
A register bank contains a number of register files or pages. Only one page can be active at a time, and there are additional instructions added to the ISA to switch between the available register pages. Data values can only be written to and read from the currently active register page, but instructions can exist to move data from one page to another.
As can be seen in this image, the gray box represents the current page, and the page can be moved up and down on the register bank.
If the register bank has N registers, and a page can only show M registers (with N > M), we can address registers with two values, n and m respectively. We can define these values as:
- n = log2(N)
- m = log2(M)
In other words, n and m are the number of bits required to address N and M registers, respectively. We can break down the address into a single value as such:
Where p is the number of bits reserved to specify the current register page. As we can see from this graphic, the current register address is simply the concatenation of the page address and the register address.
