CPSC 2310 - DAY 7
AUGUST 31, 2016

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One goal of instruction set design is to minimize instruction length

Many instructions were designed with compilers in mind.

Instruction Length
	.Affected by:
		.Memory size
		.Memory organization
		.Bus structure
		.CPU complexity
		.CPU speed

Trade offs:
		Large instruction sets vs. small programs
		Small instruction sets vs. large programs

		Large memory vs. longer instructions



Allocation of bits

Determines several important factors
.Register versus memory
	.Tradeoff between # Registers and program size
	.Studies suggest optimal number between 8 and 32
	.Most new architectures have 32 or more
	.x86 architecture allows some computation in memory

.Address range
	.large address space requires large instructions for direct addressing
	.many architectures have some restricted or short forms of displacment 
	addressing
		.ex PowerPC 16-bit displacement addressing

	.Address granularity
		.Size of object addressed
		.Typicallly 8, 16, 32 or 64

Commmon addressing modes
*	.Immediate
*	.Direct
*	.Indirect
	.Register
	.Register indirect
	.Displacement
	.Implied (stack)

Know that Accumulator machine has one main register

Know that the stack based machine doesn't have registers, operands come from
stack

Load/Store architecture - only load or store are allowed to access memory

Know Immediate, Direct, and Indirect addressing

Immediate Addressing
	.the instruction itself contains the value to be used, located in the 
	address field of the instruction

	.the value is stored in memory immediately after the instruction opcode

	.similar to using a constant in HLL

	Instruction

	+---+------------+
	|   | operand    |
	+---+------------+

Direct Addressing
	.The instruction tells where the value can be found, but the value 
	itself is out in memory.
	.The address field contains the address of the operand Effective
	address(EA) = address field (A)

	.In a HLL, direct addressing is used for global variables

         Instruction
	+----------------+
	|  | A           |
	+----------------+ -     Memory
			   |  +-----------------+
			   |  |                 |
			   |  |                 |
			   |  |			|
			   |  |			|
			   |  +-----------------+
			   ---| operand         |
			      +-----------------+


Indirect Addressing
	.The address is a pointer which holds another address
	

Register Addressing
-------------------

	.There is a very limited number of addresses
	.A very small address field is needed
	.Shorter instructions
	.No memory access needed to fetch EA
	.Very fast execution

Register-Indirect Addressing
	.Similar to memory-indirect

Indexed Addressing
------------------

A = Base
R = displacement
EA = A + R

Good for accessing arrays
EA = A + R
R++

The ARM architecture provides preindexed and postindexed addressing

Good for iterating through arrays