7
Chapter 1 Architectural Overview
The 3–bus microprocessors are dependent on burst–mode addressing to free–up
the address bus after a new instruction fetch sequence has been established. The
memory system is required to supply instructions at sequential addresses without the
processor supplying any further address information; at least until a jump or call type
instruction is executed. This makes the address bus free for use in data memory ac-
cess.
The non 3–bus processors can not simultaneously support instruction fetching
and data access from external memory. Consequently the address bus continually
supplies address information for the instruction or data access currently being sup-
ported by the external memory. However, burst–mode access signals are still sup-
plied by the processor. Indicating that the processor will require another access at the
next sequential address, after the current access is complete, is an aid in achieving
maximum memory access bandwidth. There are also a number of memory devices
available which are internally organized to give highest performance when accessed
in burst–mode.
1.3
THE Am29000 3–BUS MICROPROCESSOR
The Am29000
processor is pin compatible with other 3–bus members of the
family (see Table 1-1) [AMD 1989][Johnson 1987]. It was the first member of the
family, introduced in 1987. It is the core processor for many later designs, such as the
current 2–bus processor product line. Much of this book describes the operation of
the Am29000 processor as the framework for understanding the rest of the family.
The processor can be connected to separate Instruction and data memory sys-
tems, thus exploiting the Harvard architectural advantages (See Figure 1-4). Alter-
natively, a simplified 2–bus system can be constructed by connecting the data and
address busses together; this enables a single memory system to be constructed.
When the full potential of the 3–bus architecture is utilized, it is usually necessary to
include in the memory system a
bridge
to enable instruction memory to be accessed.
The processor does not support any on–chip means to transfer information on the
instruction bus to the data bus.
The load and store instructions, used for all external memory access, have an
option field (OPT2–0) which is presented to device pins during the data transfer op-
eration. Option field value OPT=4 is defined to indicate the bridge should permit
ROM space to be read as if it were data. Instructions can be located in two separate
spaces: Instruction space and ROM space. Often these spaces become the same, as
the IREQT pin (instruction request type) is not decoded so as to enable distinction
between the two spaces. When ROM and Instruction spaces are not common, a range
of data memory space can be set aside for accessing Instruction space via the bridge.
It is best to avoid overlapping external address spaces if high level code is to access
any memory located in the overlapping regions (see section 1.10.4).