Hardware Reference
In-Depth Information
This description of the Cortex A9 is far from complete but should give a rea-
sonable idea of how it works and how it differs from the Core i7 microarchitecture.
4.6.3 The Microarchitecture of the ATmega168 Microcontroller
Our last example of a microarchitecture is the Atmel ATmega168, shown in
Fig. 4-50. This one is considerably simpler than that of the Core i7 and
OMAP4430. The reason is that the chip must be very small and cheap to serve the
embedded design market. As such, the primary design goal of the ATmega168 was
to make the chip cheap, not fast. Cheap and Simple are good friends. Cheap and
Fast are not good friends.
Main bus 8-bit
Program
counter
Status
and control
Flash
program
memory
Interrupt
unit
32×8
general
purpose
registers
Instruction
register
SPI
unit
Watchdog
timer
Instruction
decoder
ALU
Analog
comparator
Control lines
I/O module 1
Data
SRAM
I/O module 2
I/O module 3
EEPROM
Figure 4-50. The microarchitecture of the ATmega168.
The heart of the ATmega168 is the 8-bit main bus. Attached to it are the regis-
ters and status bits, ALU, memory, and I/O devices. Let us briefly describe them
now. The register file contains 32 8-bit registers, which are used to store tempo-
rary program values. The status and control register holds the condition codes of
the last ALU operation (i.e., sign, overflow, negative, zero, and carry), plus a bit
 
 
 
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