Assembly Language Programming - Fundamentals of Computer Organization and Architecture - page 48

finally examples showing how to use the assembly language of the system in
programming sample real-life problems.
In the late 1970s, Intel introduced the 8086 as its first 16-bit microprocessor.
This processor has a 16-bit external bus. The 8086 evolved into a series of faster
and more powerful processors starting with the 80286 and ending with the Pentium.
The latter was introduced in 1993. This Intel family of processors is usually called
the X86 family. Table 3.7 summarizes the main features of the main members of
such a family.
The Intel Pentium processor has about three million transistors and its compu-
tational power ranges between two and five times that of its predecessor processor,
the 80486. A number of new features were introduced in the Pentium processor,
among which is the incorporation of a dual-pipelined superscalar architecture
capable of processing more than one instruction per clock cycle.
The basic programming model of the 386, 486, and the Pentium is shown in
Figure 3.6. It consists of three register groups. These are the general purpose regis-
ters, the segment registers, and the instruction pointer (program counter) and the flag
register. The first set consists of general purpose registers A, B, C, D, SI (source
index), DI (destination index), SP (stack pointer), and BP (base pointer). It should
be noted that in naming these registers, we used X to indicate eXtended. The
second set of registers consists of CS (code segment), SS (stack segment), and
four data segment registers DS, ES, FS, and GS. The third set of registers consists
of the instruction pointer (program counter) and the flags (status) register. The
latter is shown in Figure 3.7. Among the status bits shown in Figure 3.7, the first
five are identical to those bits introduced as early as in the 8085 8-bit microproces-
sor. The next 6-11 bits are identical to those introduced in the 8086. The flags in the
bits 12-14 were introduced in the 80286 while the 16-17 bits were introduced in
the 80386. The flag in bit 18 was introduced in the 80486. Table 3.8 shows the mean-
ing of those flags.
In the X86 family an instruction can perform an operation on one or two oper-
ands. In two-operand instructions, the second operand can be immediate data in

TABLE 3.7 Main Features of the Intel X86 Microprocessor Family

Feature 8086 286 386 486 Pentium

Date introduced 1978 1982 1985 1991 1993

Data bus 8 bits 16 bits 32 bits 32 bits 64 bits

Address bus 20 bits 24 bits 32 bits 32 bits 32 bits

Operating speed 5,8,10 MHz 6,8,10, 12.5,

16, 20 MHz

16, 20,25, 33,
40, 50 MHz

25, 33,
50 MHz

50, 60, 66,
100 MHz

Instruction cache
size

NA NA 16 bytes 32 bytes 8 Kbytes

Data cache size NA NA 256 bytes 8 Kbytes 8 Kbytes

Physical memory 1 Mbytes 16 Mbytes 4 Gbytes 4 Gbytes 4 Gbytes

Data word size 16 bits 16 bits 16 bits 32 bits 32 bits