Computer Architecture Lab/SS2013/Group3/Assignment1

Description of the microcontroller architectures

AVR

Although the AVR architecture comes in 3 main families of microcontrollers, plus other more specialized series, we will be discussing the AVR32 architecture. This architecture is a reduced instruction set computer (RISC) with a 32-bit datapath

Architecture

• data path on 32 bits
• 3 to 7 stage pipeline
• 15 general purpose registers
• Big endianness
• non-volatile flash memory for the program memory (has the advantage that the microcontroller keeps its program even after a power outage)
• data memory consists of the register file, the I/O registers (for controlling the different peripherals on the microcontroller) and a SRAM memory
• almost all AVR microcontroller have an internal EEPROM, which is for semi-permanent data storage
• execution stage works in register-to-register mode

Instruction Set

Atmel AVR instruction set overview, courtesy of Wikipedia

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0	Instruction
0	0	0	0	0	0	0	1	d d d d				r r r r				MOVW Move register pair
0	0	0	0	0	0	1			d d d				r r r			Signed and fractional multiply (R16–R23 only)
0	0	0	0	0	1	r	d d d d d					r r r r				2-operand instructions CPC, SBC, ADD, CPSE, CP, SUB. ADC, AND, EOR, OR, MOV
0	0	0	0	1
0	0	0	1
0	0	1	0
0	0	1	1	K K K K				h h h h				K K K K				Register-immediate instructions CPI, SBCI, SUBI, ORI, ANDI
0	1
1	0	k	0	k k		s	d d d d d					y	k k k			LDD/STD to Z+k or Y+k
1	0	0	1	0	0	s	d d d d d									LD/ST other
1	0	0	1	0	1	0	d d d d d					0				1-operand instructions (COM, NEG, SWAP, etc.)
1	0	0	1	0	1	0	0	B̅	b b b			1	0	0	0	SEx/CLx Status register clear/set bit
1	0	0	1	0	1	0	1					1	0	0	0	Misc instructions (RET, RETI, SLEEP, etc.)
1	0	0	1	0	1	0	c	0	0	0		1	0	0	1	Indirect jump/call to Z or EIND:Z
1	0	0	1	0	1	0	d d d d d					1	0	1	0	DEC Rd
1	0	0	1	0	1	0	0	k k k k				1	0	1	1	DES round k
1	0	0	1	0	1	0	k k k k k					1	1	c	k	JMP/CALL abs22
1	0	0	1	0	1	1		k k		p p		k k k k				ADIW/SBIW Rp,uimm6
1	0	0	1	1	0	B		a a a a a					b b b			I/O space bit operations
1	0	0	1	1	1	r	d d d d d					r r r r				MUL, unsigned: R1:R0 = Rr×Rd
1	0	k	0	k k		s	d d d d d					y	k k k			See 10k0 above
1	0	1	1	s	a a		d d d d d					a a a a				OUT/IN to I/O space
1	1	0	c	12 bit signed offset												Relative jump/call to PC ± 2×simm12
1	1	1	0	K K K K				h h h h				K K K K				LDI Rh,K
1	1	1	1	0	B̅	7-bit signed offset							b b b			Conditional branch on status register bit
1	1	1	1	1	0	s	d d d d d					0	b b b			BLD/BST register bit to STATUS.T
1	1	1	1	1	1	B	d d d d d					0	b b b			SBRC/SBRS skip if register bit equals B

ARM

Based on the high market share of the ARM-based chips, we can safely say that it is the most used reduced instruction set computer (RISC) today.

Architecture

• data path on 32 or 64 bits
• 3 stage pipeline (up to ARM7 version), up to 13 stages for higher performance versions
• 16 general purpose registers, some having special function (e.g. Stack Pointer, Link Register, Program Counter)
• Bi-endianness, with little endian as default
• support for 16 coprocessors, connected in a non-intrusive way
• floating point based on VPF (Vector Floating Point) technology, as coprocessor
• execution stage works in register-to-register mode

Instruction Set

• ARM relies on a load/store architecture
• 32-bit wide instructions
• mostly single clock cycle execution
• link register for fast leaf function calls
• 2 priority level interrupt subsystem
• supports add, subtract and multiply instructions, with some cores implementing integer divide
• fold shifts/rotates into the data processing, reducing memory accesses and therefore improving pipeline efficiency

ARM resources

MIPS

MIPS (originally an acronym for Microprocessor without Interlocked Pipeline Stages) is a reduced instruction set computer (RISC) instruction set architecture (ISA) developed by MIPS Technologies (formerly MIPS Computer Systems, Inc.). Early versions had a 32-bit wide datapath, with latter ones at 64-bit.

Architecture (considering R2000 version)

Pipeline diagram of the MIPS architecture - courtesy of Wikipedia

• data path on 32 bits
• 5 pipeline stages:

1. Instruction Fetch
2. Instruction Decode
3. Execute
4. Memory read/store
5. Writeback

• 32 32-bit general purpose registers (GPR)
• could be booted either as big-endian or little-endian
• support for up to 4 co-processors
• could plug in the R2010 Floating Point Unit (FPU), having 32 32-bit registers (which could be used as 16 64-bit registers, for double precision)
• external SRAM cache
• internal DRAM (very slow writing time)
• execution stage is of type register-to-register, the ALU obtaining values from the register file and writing back to the register file during writeback stage

Instruction Set

We are assuming an 32-bit MIPS processor, therefore the instruction size is of 32 bits. The instruction set is based around 3 types of instructions:

• R - for operations based around values in the registers (arithmetic, logical, data transfer, unconditional jump register)
• I - for operations based around values in the registers as well as an immediate value
• J - for jump instructions, having only an immediate value specified (due to the need of all 26 bits for the address space)

Instruction formats are as follows:

Type	31 format (bits) 0
R	opcode (6)	rs (5)	rt (5)	rd (5)	shamt (5)	funct(6)
I	opcode (6)	rs (5)	rt (5)	immediate (16)
J	opcode (6)	address (26)

For the complete list of instructions check this link.