Geode™ GXm Processor
1.0
Architecture Overview
The instruction fetch pipeline stage generates, from the
on-chip cache, a continuous high-speed instruction
stream for use by the processor. Up to 128 bits of code
are read during a single clock cycle.
Branch prediction logic within the prefetch unit generates
a predicted target address for unconditional or conditional
branch instructions. When a branch instruction is
detected, the instruction fetch stage starts loading instruc-
tions at the predicted address within a single clock cycle.
Up to 48 bytes of code are queued prior to the instruction
decode stage.
The instruction decode stage evaluates the code stream
provided by the instruction fetch stage and determines the
number of bytes in each instruction and the instruction
type. Instructions are processed and decoded at a maxi-
mum rate of one instruction per clock.
The address calculation function is super-pipelined and
contains two stages, AC1 and AC2. If the instruction
refers to a memory operand, AC1 calculates a linear
memory address for the instruction.
The AC2 stage performs any required memory manage-
ment functions, cache accesses, and register file
accesses. If a floating point instruction is detected by
AC2, the instruction is sent to the floating point unit for
processing.
The execution stage, under control of microcode, exe-
cutes instructions using the operands provided by the
address calculation stage.
Write-back, the last stage of the integer unit, updates the
register file within the integer unit or writes to the
load/store unit within the memory management unit.
The National Semiconductor Geode GXm processor is
an x86-compatible 32-bit microprocessor. The decoupled
load/store unit (within the memory management unit)
allows multiple instructions in a single clock cycle. Other
features include single-cycle execution, single-cycle
instruction decode, 16 KB write-back cache, and clock
rates up to 266 MHz. These features are made possible
by the use of advanced-process technologies and super-
pipelining.
The GXm processor has low power consumption at all
clock frequencies. Where additional power savings are
required, designers can make use of Suspend mode, Stop
Clock capability, and System Management Mode (SMM).
The GXm processor is divided into major functional blocks
(as shown in Figure 1-1):
• Integer Unit
• Floating Point Unit (FPU)
• Write-Back Cache Unit
• Memory Management Unit (MMU)
• Internal Bus Interface Unit
• Integrated Functions
Instructions are executed in the integer unit and in the
floating point unit. The cache unit stores the most recently
used data and instructions and provides fast access to
this information for the integer and floating point units.
1.1
INTEGER UNIT
The integer unit consists of:
• Instruction Buffer
• Instruction Fetch
• Instruction Decoder and Execution
The superpipelined integer unit fetches, decodes, and
executes x86 instructions through the use of a six-stage
integer pipeline.
Write-Back
Cache Unit
C-Bus
MMU
Integer
Unit
FPU
Internal Bus Interface Unit
X-Bus
Integrated
Functions
Graphics
Pipeline
Memory
Controller
Display
Controller
PCI
Controller
SDRAM Port
CS5530
(CRT/LCD TFT)
PCI Bus
Figure 1-1. Internal Block Diagram
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