Intel 386 manual Direct Mapped Cache Organization

CACHE SUBSYSTEMS

Each direct mapped Cilche address has two parts. The first part, called the cache index field, contains enough bits to specify a block location within the cache. The second part, called the tag field, contains enough bits to distinguish a block from other blocks that

.may be stored at a particular cache location.

For example, consider a 64-kilobyte direcf mapped cache that contains 16K 32-bit loca- tions and caches 16 megabytes of main memory. The cache index field must include 14 bits to select one of the 16K blocks in the cache, plus 2 bits (or 4 byte Enables) to select a byte from the 4-byte block. The tag field must be 8 bits wide to identify One of the 256 blocks that can occupy the seleCted cache location. The remaining 8 bits of the 32-bit Intel386 DX microprocessor address are decoded to select the cache subsystem from among other memories in the memory space. The direct-mapped cache organiza- tion is shown in Figure 7-3.

32.BIT	131		24123	. 16	15			o
PROCESSOR	ICACHE/DRAM I					INDEX
ADDRESS	SE~ECT		TAG
					j.-64K CACHE = 16 BITS--+!
			1+--16 MEGABYTE DRAM = 24 BITS----+!
							DATA	INDEX	TAG
								FFFC
							11223344	FFF8
INDEX	TAG		DATA					0010	FF
								0000
									I
FFFC	01		12345678					0008
				f...J				0004
FFF8	FF		11223344
							--	0000
0010
OOOC			87654321	P			-		I
0008	00				'---		12345678	FFFC
0004	01		11235813
								FFF8
0000	00		13579246	I-	-
(14 BITS) j.8 BIT~		j+32BITS~						0010	01
								OOOC
								0008	I~
	64KSRAM CACHE						11235813	0004
								0000
								FFFC
								FFF8
								.0010
								OOOC
							87654321	0008
								0004
							13579246	0000

j+32BITS.j

16 MEGABYTE DRAM

231732;7·3

Figure 7-3. Direct Mapped Cache Organization

7-5