UPSD3422 datasheet(24/293 Pages) STMICROELECTRONICS

24 / 293 page

8032 MCU core performance enhancements

uPSD34xx

24/293

Figure 8.

Instruction Pre-Fetch Queue and Branch Cache

5.1

Pre-fetch queue (PFQ) and branch cache (BC)

The PFQ is always working to minimize the idle bus time inherent to 8032 MCU architecture,

to eliminate wasted memory fetches, and to maximize memory bandwidth to the MCU. The

PFQ does this by running asynchronously in relation to the MCU, looking ahead to pre-fetch

two bytes (word) of code from program memory during any idle bus periods. Only necessary

word will be fetched (no dummy fetches like standard 8032). The PFQ will queue up to four

code bytes in advance of execution, which significantly optimizes sequential program

performance. However, when program execution becomes non-sequential (program

branch), a typical pre-fetch queue will empty itself and reload new code, causing the MCU to

stall. The Turbo uPSD34xx diminishes this problem by using a Branch Cache with the PFQ.

The BC is a four-way, fully associative cache, meaning that when a program branch occurs,

its branch destination address is compared simultaneously with four recent previous branch

destinations stored in the BC. Each of the four cache entries contain up to four bytes of code

related to a branch. If there is a hit (a match), then all four code bytes of the matching

program branch are transferred immediately and simultaneously from the BC to the PFQ,

and execution on that branch continues with minimal delay. This greatly reduces the chance

that the MCU will stall from an empty PFQ, and improves performance in embedded control

systems where it is quite common to branch and loop in relatively small code localities.

By default, the PFQ and BC are enabled after power-up or reset. The 8032 can disable the

PFQ and BC at runtime if desired by writing to a specific SFR (BUSCON).

The memory in the PSD module operates with variable wait states depending on the value

specified in the SFR named BUSCON. For example, a 5V uPSD34xx device operating at a

40MHz crystal frequency requires four memory wait states (equal to four MCU clocks). In

this example, once the PFQ has one word of code, the wait states become transparent and

a full 10 MIPS is achieved when the program stream consists of sequential one- or two-byte,

one machine-cycle instructions as shown in Figure 7 on page 23 (transparent because a

machine-cycle is four MCU clocks which equals the memory pre-fetch wait time that is also

Branch 4

Code

Branch 4

Code

8032

MCU

16-bit

Program

Memory

on PSD

Module

Instruction Pre-Fetch Queue (PFQ)

4 Bytes of Instruction

Instruction Byte

Wait

Instruction Byte

Current

Branch

Address

Compare

Branch

Cache

(BC)

AI10431

Address

Load on Branch Address Match

Branch 3

Code

Branch 3

Code

Branch 2

Code

Branch 2

Code

Branch 1

Code

Branch 1

Code

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