15

0

TAG (7)

TAG (6)

TAG (5)

TAG (4)

TAG (3)

TAG (2)

TAG (1)

TAG (0)

NOTE

The index i of tag(i) is not top-relative A program typically uses the ‘‘top’’ field of Status Word to determine which tag(i)

field refers to logical top of stack

TAG VALUES

23 Register Set

Figure 11 shows the Intel387 DX MCP register set

When an MCP is present in a system programmers

may use these registers in addition to the registers

normally available on the Intel386 DX CPU

231 DATA REGISTERS

Intel387 DX MCP computations use the MCP’s data

registers These eight 80-bit registers provide the

equivalent capacity of twenty 32-bit registers Each

of the eight data registers in the MCP is 80 bits wide

and is divided into ‘‘fields’’ corresponding to the

MCPs extended-precision real data type

The Intel387 DX MCP register set can be accessed

either as a stack with instructions operating on the

top one or two stack elements or as a fixed register

set with instructions operating on explicitly designat-

ed registers The TOP field in the status word identi-

fies the current top-of-stack register A ‘‘push’’ oper-

ation decrements TOP by one and loads a value into

the new top register A ‘‘pop’’ operation stores the

value from the current top register and then incre-

ments TOP by one Like the Intel386 DX Microproc-

essor stacks in memory the MCP register stack

grows ‘‘down’’ toward lower-addressed registers

Instructions may address the data registers either

implicitly or explicitly Many instructions operate on

the register at the TOP of the stack These instruc-

tions implicitly address the register at which TOP

points Other instructions allow the programmer to

explicitly specify which register to user This explicit

register addressing is also relative to TOP

232 TAG WORD

The tag word marks the content of each numeric

data register as Figure 21 shows Each two-bit tag

represents one of the eight numerics registers The

principal function of the tag word is to optimize the

MCPs performance and stack handling by making it

possible to distinguish between empty and nonemp-

ty register locations It also enables exception han-

dlers to check the contents of a stack location with-

out the need to perform complex decoding of the

actual data

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