ARM Architecture
Computer Organization and Assembly Languages
Mr. Surendra Mehra
ARM history
• 1983 developed by Acorn computers
– To replace 6502 in BBC computers
– 4-man VLSI design team
– Its simplicity comes from the inexperience team
– Match the needs for generalized SoC for reasonable
power, performance and die size
– The first commercial RISC implemenation
• 1990 ARM (Advanced RISC Machine), owned by
Acorn, Apple and VLSI
ARM Ltd
Design and license ARM core design but not fabricate
Why ARM?
• One of the most licensed and thus widespread
processor cores in the world
– Used in PDA, cell phones, multimedia players,
handheld game console, digital TV and cameras
– ARM7: GBA, iPod
– ARM9: NDS, PSP, Sony Ericsson, BenQ
– ARM11: Apple iPhone, Nokia N93, N800
– 90% of 32-bit embedded RISC processors till 2009
• Used especially in portable devices due to its
low power consumption and reasonable
performance
ARM powered products
ARM processors
• A simple but powerful design
• A whole family of designs sharing similar design
principles and a common instruction set
Naming ARM
• ARMxyzTDMIEJFS
– x: series
– y: MMU
– z: cache
– T: Thumb
– D: debugger
– M: Multiplier
– I: EmbeddedICE (built-in debugger hardware)
– E: Enhanced instruction
– J: Jazelle (JVM)
– F: Floating-point
– S: Synthesizible version (source code version for EDA
tools)
Popular ARM architectures
• ARM7TDMI
– 3 pipeline stages (fetch/decode/execute)
– High code density/low power consumption
– One of the most used ARM-version (for low-end
systems)
– All ARM cores after ARM7TDMI include TDMI even if
they do not include TDMI in their labels
• ARM9TDMI
– Compatible with ARM7
– 5 stages (fetch/decode/execute/memory/write)
– Separate instruction and data cache
• ARM11
ARM family comparison
year 1995 1997 1999 2003
ARM is a RISC
• RISC: simple but powerful instructions that
execute within a single cycle at high clock speed.
• Four major design rules:
– Instructions: reduced set/single cycle/fixed length
– Pipeline: decode in one stage/no need for microcode
– Registers: a large set of general-purpose registers
– Load/store architecture: data processing instructions
apply to registers only; load/store to transfer data
from memory
• Results in simple design and fast clock rate
• The distinction blurs because CISC implements
RISC concepts
ARM design philosophy
• Small processor for lower power consumption
(for embedded system)
• High code density for limited memory and
physical size restrictions
• The ability to use slow and low-cost memory
• Reduced die size for reducing manufacture cost
and accommodating more peripherals
ARM features
• Different from pure RISC in several ways:
– Variable cycle execution for certain instructions:
multiple-register load/store (faster/higher code
density)
– Inline barrel shifter leading to more complex
instructions: improves performance and code density
– Thumb 16-bit instruction set: 30% code density
improvement
– Conditional execution: improve performance and
code density by reducing branch
– Enhanced instructions: DSP instructions
ARM architecture
ARM architecture
• Load/store
architecture
• A large array of
uniform registers
• Fixed-length 32-bit
instructions
• 3-address instructions
Registers
• Only 16 registers are visible to a specific mode.
A mode could access
– A particular set of r0-r12
– r13 (sp, stack pointer)
– r14 (lr, link register)
– r15 (pc, program counter)
– Current program status register (cpsr)
– The uses of r0-r13 are orthogonal
General-purpose registers
31 24 23 16 15 87 0
8-bit Byte
16-bit Half word
32-bit word
• 6 data types (signed/unsigned)
• All ARM operations are 32-bit. Shorter data
types are only supported by data transfer
operations.
Program counter
• Store the address of the instruction to be
executed
• All instructions are 32-bit wide and word-
aligned
• Thus, the last two bits of pc are undefined.
Program status register (CPSR)
mode bits
overflow Thumb state
carry/borrow FIQ disable
zero IRQ disable
negative
Processor modes
Register organization
Instruction sets
• ARM/Thumb/Jazelle
Pipeline
ARM7
ARM9
In execution, pc always 8 bytes ahead
Pipeline
• Execution of a branch or direct modification of
pc causes ARM core to flush its pipeline
• ARM10 starts to use branch prediction
• An instruction in the execution stage will
complete even though an interrupt has been
raised. Other instructions in the pipeline are
abondond.
Interrupts
Vector table
Interrupt
handlers
code
Interrupts
References
