C8051F360 datasheet(145/288 Pages) SILABS

145 / 288 page

Rev. 1.0

145

C8051F360/1/2/3/4/5/6/7/8/9

14. Branch Target Cache

The C8051F36x device families incorporate a 32x4 byte branch target cache with a 4-byte prefetch

engine. Because the access time of the Flash memory is 40 ns, and the minimum instruction time is 10 ns

(C8051F360/1/2/3/4/5/6/7) or 20 ns (C8051F368/9), the branch target cache and prefetch engine are nec-

essary for full-speed code execution. Instructions are read from Flash memory four bytes at a time by the

prefetch engine, and given to the CIP-51 processor core to execute. When running linear code (code with-

out any jumps or branches), the prefetch engine alone allows instructions to be executed at full speed.

When a code branch occurs, a search is performed for the branch target (destination address) in the

cache. If the branch target information is found in the cache (called a “cache hit”), the instruction data is

read from the cache and immediately returned to the CIP-51 with no delay in code execution. If the branch

target is not found in the cache (called a “cache miss”), the processor may be stalled for up to four clock

cycles while the next set of four instructions is retrieved from Flash memory. Each time a cache miss

occurs, the requested instruction data is written to the cache if allowed by the current cache settings. A

data flow diagram of the interaction between the CIP-51 and the Branch Target Cache and Prefetch

Engine is shown in Figure 14.1.

Flash

Memory

Branch Target

Cache

Prefetch

Engine

Instruction

Data

CIP-51

Instruction Address

Figure 14.1. Branch Target Cache Data Flow

14.1. Cache and Prefetch Operation

The branch target cache maintains two sets of memory locations: “slots” and “tags”. A slot is where the

cached instruction data from Flash is stored. Each slot holds four consecutive code bytes. A tag contains

the 13 most significant bits of the corresponding Flash address for each four-byte slot. Thus, instruction

data is always cached along four-byte boundaries in code space. A tag also contains a “valid bit”, which

indicates whether a cache location contains valid instruction data. A special cache location (called the lin-

ear tag and slot), is reserved for use by the prefetch engine. The cache organization is shown in

Figure 14.2. Each time a Flash read is requested, the address is compared with all valid cache tag loca-

tions (including the linear tag). If any of the tag locations match the requested address, the data from that

slot is immediately provided to the CIP-51. If the requested address matches a location that is currently

being read by the prefetch engine, the CIP-51 will be stalled until the read is complete. If a match is not

found, the current prefetch operation is abandoned, and a new prefetch operation is initiated for the

requested instruction data. When the prefetch operation is finished, the CIP-51 begins executing the

instructions that were retrieved, and the prefetch engine begins reading the next four-byte word from Flash

memory. If the newly-fetched data also meets the criteria necessary to be cached, it will be written to the

cache in the slot indicated by the current replacement algorithm.

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