2. Introduction
Words of a computer’s language are called its instructions
Its vocabulary is its instruction set.
Goal:
Find a language that makes it easy to build the hardware and the compiler,
while maximizing performance and minimizing cost

3. Review: Instruction Set Design
software
instruction set
hardware
Which is easier to change?

4. Stored Program Computer
Basic Principles
Use of instructions that are indistinguishable from numbers
Use of alterable memory for programs
Demands balance among number of instructions, the number of clock cycles needed by an instruction and the speed of the clock.

5. Instruction Set Architecture: What Must be Specified?
° Instruction Format or Encoding
– how is it decoded?
° Location of operands and result
– where other than memory?
– how many explicit operands?
– how are memory operands located?
– which can or cannot be in memory?
° Data type and Size
° Operations
– what are supported
° Successor instruction
– jumps, conditions, branches
Instruction
Fetch
Instruction
Decode
Operand
Fetch
Execute
Result
Store
Next
Instruction
- fetch-decode-execute is implicit!

6. Overview of Design Principles
1. Simplicity favors regularity
keep all instructions a single size
require three register operands for arithmetic
keep register fields in same place in each instruction
2. Smaller is faster
the reason that MIPS has 32 registers rather than many more
3.Make the common case fast
PC-relative addressing for conditional branch
immediate addressing for constant operands
4.Good design demands good compromises
compromise between larger addresses and keeping instructions same length

7. MIPS I
Instruction set

8. MIPS Typical of instruction sets since 1980’s
Almost 100 million MIPS processors manufactured in 2002
Found in products from Cisco, NEC, Nintendo, Silicon Graphics, Sony, Texas Instruments, Toshiba, and others

9. Historically General Purpose Registers Dominate
all machines use general purpose registers
Advantages of registers •
registers are faster than memory •
registers are easier for a compiler to use -
e.g., (A*B) – (C*D) – (E*F) can do multiplies in any order
vs. stack •
registers can hold variables -
memory traffic is reduced, so program is sped up
(since registers are faster than memory) -
code density improves (since register named with fewer bits
than memory location)

10. MIPS Architecture
MIPS – semiconductor company that built one of the first commercial RISC architectures
We will study the MIPS architecture in some detail in this class Why MIPS instead of Intel 80x86?
MIPS is simple, elegant. Don’t want to get bogged down in gritty details.
MIPS widely used in embedded apps, x86 little used in embedded, and more embedded computers than PCs

11. Instruction Set Architecture
Early trend was to add more and more instructions to new CPUs to do elaborate operations
VAX architecture had an instruction to multiply polynomials!
RISC philosophy (Cocke IBM, Patterson, Hennessy,1980s)–Reduced Instruction Set Computing
Keep the instruction set small and simple, makes it easier to build fast hardware.
Let software do complicated operations by composing simpler ones.

12. Operations of the Computer Hardware
The MIPS assembly language instruction
add a, b, c means a = b+c
This sequence adds four variables (a=b+c+d+e)
add a, b, c #the sum of b and c is placed in a
add a, a, d #the sum of b,c, and d is now in a
add a, a, e #the sum of b,c,d and e is now in a
Notice that it takes 3 instructions to add four variables

13. MIPS Instructions Design Principle 1: Simplicity favors regularity
The MIPS assembly language instruction
add a, b, c means a = b+c
Each line represents one instruction
Each instruction has exactly 3 operands for simplicity
There is one operation per MIPS instruction
Instructions are related to operations (=, +, -, *, /)
in C or Java

14. Operands of the Computer Hardware
Operands of arithmetic instructions must be from a limited number of special memory locations called registers
Size of a MIPS register is 32 bits - called a word
Major difference between variables in programming language (unlimited) and registers is the limited number of registers- typically 32 in MIPS

15. C, Java Variables vs. Registers
In C (and most High Level Languages) variables are declared first and given a type
Example: int fahr, celsius; char a, b, c, d, e;
Each variable can ONLY represent a value of the type it was declared as (cannot mix and match int and char variables).
In Assembly Language, the registers have no type; operation determines how register contents are treated

16. MIPS I Registers Programmable storage 232 x bytes of memory
31 x 32-bit General Purpose Registers GPRs (R0 = 0)

17. Operands of the Computer Hardware
Design Principle 2: Smaller is faster
Very large number of registers may increase clock cycle time because it takes electronic signals longer to travel farther.
Using more than 32 registers would require a different instruction format.
MIPS register convention is to use two character names following a dollar sign:
$s0, $s1… for variables
$t0, $t1… for temporary locations
$a0, $a1…for arguments

18. MIPS Addition and Subtraction
Syntax of Instructions:
1 2,3,4
where:
1) operation by name
2) operand getting result (“destination”)
3) 1st operand for operation (“source1”)
4) 2nd operand for operation (“source2”)
Syntax is rigid:
1 operator, 3 operands
Why? Keep Hardware simple via regularity

19. MIPS Addition and Subtraction of Integers
Addition in Assembly
Example: add $s0,$s1,$s2 (in MIPS)
Equivalent to: a = b + c (in C/Java)
where MIPS registers $s0,$s1,$s2 are associated with C variables a, b, c
Subtraction in Assembly
Example: sub $s3,$s4,$s5 (in MIPS)
Equivalent to: d = e - f (in C)
where MIPS registers $s3,$s4,$s5 are associated with C variables d, e, f

20. Addition and Subtraction
How would MIPS do this C/Java statement?
a = b + c + d - e;
Break into multiple instructions
add $t0, $s1, $s2 # temp = b + c
add $t0, $t0, $s3 # temp = temp + d
sub $s0, $t0, $s4 # a = temp - e
Notice: A single line of C or Java may break up into several lines of MIPS. Also everything after the hash mark- # - on each line is ignored (comments)

21. Addition and Subtraction
How do we do this?
f = (g + h) - (i + j);
Use intermediate temporary register
add $t0,$s1,$s2 # temp0 = g + h
add $t1,$s3,$s4 # temp1 = i + j
sub $s0,$t0,$t1 # f=(g+h)-(i+j)

22. Constants or Immediate Operands
Design Principle 3: Make the common case FAST
Constants occur frequently and by including constants in arithmetic instructions, they are faster than if the constants were loaded from memory
To add 4 to register 3 use add immediate (addi):
addi $s3,$s3, # $s3 = $s3+4

23. Immediates Immediates are numerical constants.
They appear often in code, so there are special instructions for them.
Add Immediate:
addi $s0,$s1,10 (in MIPS)
f = g + 10 (in C)
where MIPS registers $s0,$s1 are associated with C or Java variables f, g
Syntax similar to add instruction, except that the
last argument is a number instead of a register.

24. Immediates and Subtraction
There is no Subtract Immediate in MIPS: Why?
Limit types of operations that can be done to absolute minimum
negative constants are less frequent
if an operation can be decomposed into a simpler operation, don’t include it
addi …, -X = subi …, X => no need for subi
addi $s0,$s1,-10 (in MIPS)
f = g - 10 (in C)
where MIPS registers $s0,$s1 are associated with C or Java variables f, g

25. Register Zero
One particular immediate, the number zero (0), appears very often in code.
So we define register zero ($0 or $zero) to always have the value 0; for example:
add $s0,$s1,$zero (in MIPS)
f = g (in C)
where MIPS registers $s0,$s1 are associated with C variables f, g
Defined in hardware, so an instruction
add $zero,$zero,$s0
will not do anything!

26. Summarizing... In MIPS Assembly Language: New Instructions:
Registers replace C variables
One Instruction (simple operation) per line
Simpler is Better
Smaller is Faster
New Instructions:
add, addi, sub
New Registers:
C or Java Variables: $s0 - $s7
Temporary Variables: $t0 - $t9
Zero: $zero

27. Memory Operands
Programming Languages have both simple variables and complex data structures.
How can we handle large data structures with just a few registers?
Data structures are kept in memory.
MIPS includes instructions to transfer data between memory and registers.
Data transfer instructions ( load, store)

28. Memory Operands Data transfer Instruction Format
load copies data from memory to register
lw - load word
Format
opcode register , constant (register)
memory address
Syntax
lw $t0, 8 ($s3)
offset base address

29. Memory Addressing
Since 1980 almost every machine uses addresses to level of 8-bits
(byte)
2 questions for design of Instruction Set Architecture: •
Since we could read a 32-bit word as
four loads of bytes from sequential byte addresses
or as one load word from a single byte address,
How do byte addresses map onto words? •
Can a word be placed on any byte boundary?

30. Addressing Objects: Alignment
Since 8-bit bites are useful, most architectures address individual bites.
Address of a word matches the address of one of the four bites in the word
Addresses of sequential words differ by 4 bytes
MIPS words must start at addresses that are multiples of 4 - called alignment restriction

31. Addressing Objects: Endianess
Computers are grouped into those that use:
the address of the leftmost or “big end byte” as the word address
and those that use the “little end” or rightmost byte
MIPS is in the BIG Endian group

32. Addressing Objects: Endianess and Alignment
Big Endian: address of most significant IBM 360/370, Motorola 68k, MIPS, Sparc, HP PA
Little Endian: address of least significant Intel 80x86, DEC Vax, DEC Alpha (Windows NT)
little endian byte 0
msb
lsb
Aligned
big endian byte 0
Alignment: require that objects fall
on address that is multiple of their size.
Not
Aligned

33. Addressing arrays Byte addressing also affects the array index
The offset to be added to the base register ($s3) must be 4 x 8, or 32
lw $t0, 32($s3) 12 8 4
$s3
Addresses
DATA

34. Load and Store The complementary instruction to load is store
Store copies data from a register into memory
Format similar to load
MIPS name is sw for store word
sw $t0, 48($s3) # if $s3 contains the base address # of A[0] in memory, then this # stores result into A[12]

35. Memory and Registers
Process of putting less commonly used variables in memory is called spilling the registers.
Arithmetic instructions reads two registers, operates on them and writes the result.
Data transfer instructions only reads or writes one operand, without operating on it.
MIPS register take less time and have greater throughput than memory, thus a compiler must use registers efficiently!

36. Representing Instructions Inside the Computer
Instructions are stored as a series of bits, or binary numbers:
add = $s =$s =$t0 no shift add=32
Instruction: add $t0, $s1, $s2
There must be a way to represent register names as numbers
In MIPS there is a mapping, for example
$t0… $t7 map to registers 8 …15
$s0…$s7 map to registers 16…23
000000
100001
10010
01000
100000
opcode rs rt rd
funct
shamt

37. MIPS Assembler Register Convention
“caller saved”
“callee saved”
On Green Card in Column #2 at bottom

38. Why Multiple Instruction Formats?
Design Principle 4: Good design demands good compromises -
there is a need to keep instructions the same length and desire for a single format
There is a problem using previous (R-format) when an instruction needs longer fields
for example lw must specify two registers and a constant, but the constant would have only 5 bits available, so the largest value would be 25 = 32
Solution: allow I and J formats for different instructions - but keep all the same length = 32 bits

39. Instruction Formats
I-format: used for instructions with immediates, lw and sw (since the offset counts as an immediate), and the branches (beq and bne),
(but not the shift instructions; later)
J-format: used for j and jal(jump and link)
R-format: used for all other instructions
It will soon become clear why the instructions have been partitioned in this way.

40. Comments Instruction Fields
Instruction Format Names and Field Descriptions
funct
6 bits
(5-0)
Jump instruction format
Target address op
J-format
(2 fields)
Data Transfer, branch, immediate instruction format
Address/immediate rt rs
I-format
(4 fields)
Arithmetic instruction format
shamt rd
R-format
(6 fields)
All MIPS instructions 32 bits
5 bits
(10-6)
5 bits (15-11)
(20-16)
(25-21)
(31-26)
Comments
Instruction Fields
Name
Instruction field notes:
The op and funct fields form the op-code. The rs field gives a source register and rt is also normally a source register. rd is the destination register, and shamt supplies the shift amount for logical shift operations.

41. R-Format Instructions
Define “fields” of the following number of bits each: = 32 6 5
For simplicity, each field has a name:
opcode rs rt rd
funct
shamt

42. R-Format Instructions
Meaning of fields:
rs (Source Register): generally used to specify register containing first operand
rt (Target Register): generally used to specify register containing second operand (note that name is misleading)
rd (Destination Register): generally used to specify register which will receive result of computation
shamt (Shift amount)
funct ( Function) - selects specific variant of the opcode operation - sometimes called function code

43. R-Format Example MIPS Instruction: add $8,$9,$10
Decimal number per field representation: 9 10 8 32
Binary number per field representation:
hex
000000
01001
01010
01000
100000
00000
hex representation: A 4020hex
decimal representation: ,546,144ten
On Green Card: Format in column 1, opcodes in column 3

44. J-Format Instructions
Define “fields” of the following number of bits each:
6 bits
26 bits
As usual, each field has a name:
opcode
target address
Key Concepts
Keep opcode field identical to R-format and I-format for consistency.
Combine all other fields to make room for large target address.

45. Arrays and Data Structures
C and Java variables map onto registers; what about large data structures like arrays?
1 of the 5 components of a computer - the memory- contains such data structures
But MIPS arithmetic instructions only operate on registers, never directly on memory.
Data transfer instructions transfer data between registers and memory:
Memory to register
Register to memory

46. Overview of MIPS Simple instructions all 32 bits wide
Very structured, no unnecessary baggage
Only three instruction formats rely on compiler to achieve performance — what are the compiler's goals?
help compiler where we can R I J
op rs rt rd shamt funct
op rs rt 16 bit address
op bit address

47. Anatomy: 5 components of any Computer
Registers are in the datapath of the processor; if operands are in memory, we must transfer them to the processor to operate on them, and then transfer back to memory when done.
Personal Computer
Computer
Processor
Memory
Devices
Control
(“brain”)
Input
Store (to)
Datapath
Registers
Output
Load (from)
These are “data transfer” instructions…

48. Data Transfer: Memory to Registers
To transfer a word of data, we need to specify two things:
Register: specify this by # ($0 - $31) or symbolic name ($s0,…, $t0, …)
Memory address: more difficult
Think of memory as a single one-dimensional array, so we can address it simply by supplying a pointer to a memory address.
Other times, we want to be able to offset from this pointer.
Remember: “Load FROM memory”

49. Data Transfer: Memory to Registers
To specify a memory address to copy from, specify two things:
A register containing a pointer to memory
A numerical offset (in bytes)
The desired memory address is the sum of these two values.
Example: 8($t0)
specifies the memory address pointed to by the value in $t0, plus 8 bytes

50. Data Transfer: Memory to Register
Load Instruction Syntax:
1 2, 3(4)
where
1) operation name
2) register that will receive value
3) numerical offset in bytes
4) register containing pointer to memory
MIPS Instruction Name:
lw (meaning Load Word, so 32 bits or one word are loaded at a time)

51. Data Transfer: Memory to Register
Data flow
Example: lw $t0,12($s0)
This instruction will take the pointer in $s0, add 12 bytes to it, and then load the value from the memory pointed to by this calculated sum into register $t0
Notes:
$s0 is called the base register
12 is called the offset
offset is generally used in accessing elements of array or structure: base register points to beginning of array or structure

52. Data Transfer: Register to Memory
Also want to store from register into memory
Store instruction syntax is identical to Load’s
MIPS Instruction Name:
sw (meaning Store Word, so 32 bits or one word are loaded at a time)
Example: sw $t0,12($s0)
This instruction will take the pointer in $s0, add 12 bytes to it, and then store the value from register $t0 into that memory address
Remember: “ Store INTO memory”
Data flow

53. Pointers v. Values Key Concept:
A register can hold any 32-bit value. That value can be a (signed) int, an unsigned int, a pointer (memory address), and so on
If you write add $t2,$t1,$t0 then $t0 and $t1 must contain values
If you write lw $t2,0($t0) then $t0 must contain a pointer to memory
Don’t mix these up!

54. Addressing: Byte vs. Word
Every word in memory has an address, similar to an index in an array
Early computers numbered words like C numbers elements of an array:
Memory[0], Memory[1], Memory[2], …
Called the “address” of a word
Computers needed to access 8-bit bytes as well as words (4 bytes/word)
Today machines address memory as bytes, (i.e.,“Byte Addressed”) hence 32-bit (4 byte) word addresses differ by 4
Memory[0], Memory[4], Memory[8], …

55. Compilation with Memory
What offset in lw to select A[5] in C/Java?
4x5=20 to select A[5]: byte v. word
Compile by hand using registers: g = h + A[5];
g: $s1, h: $s2, $s3:base address of A
1st transfer from memory to register:
lw $t0,20($s3) # $t0 gets A[5]
Add 20 to $s3 to select A[5], put into $t0
Next add it to h and place in g add $s1,$s2,$t0 # $s1 = h+A[5]

56. Translating Assembly Language into Machine Language
Suppose $t1 has base of array A and $s2 corresponds to h in the assignment
A[300] = h + A[300]
In MIPS : ( try this )
lw $t0, 1200 ($t1) # temp register $t0 gets A[300]
add $t0, $s2, $t # temp register $t0 gets h+ A[300]
sw $t0, 1200($t1) #stores h = A[300] back into A[300]

57. MIPS Instruction Encoding

58. Translating Assembly Language into Machine Language
lw $t0, 1200 ($t1) # temp register $t0 gets A[300]
add $t0, $s2, $t0 # temp register $t0 gets h+ A[300]
sw $t0, 1200($t1) #stores h = A[300] back into A[300]

59. Notes about Memory
Pitfall: Forgetting that sequential word addresses in machines with byte addressing do not differ by 1.
Many an assembly language programmer has toiled over errors made by assuming that the address of the next word can be found by incrementing the address in a register by 1 instead of by the word size in bytes.
So remember that for both lw and sw, the sum of the base address and the offset must be a multiple of (to be word aligned)

60. C/Java Decisions: if Statements
2 kinds of if statements in C
if (condition) clause
if (condition) clause1 else clause2
Rearrange 2nd if into following:
if (condition) goto L1; clause2; goto L2; L1: clause1;
L2:
Not as elegant as if-else, but same meaning

61. MIPS Decision Instructions
Decision instruction in MIPS:
beq register1, register2, L1
beq is “Branch if (registers are) equal” Same meaning as (using C/Java): if (register1==register2) goto L1
Complementary MIPS decision instruction
bne register1, register2, L1
bne is “Branch if (registers are) not equal” Same meaning as (using C): if (register1!=register2) goto L1
Called conditional branches

62. MIPS Goto Instruction
In addition to conditional branches, MIPS has an unconditional branch:
j label
Called a Jump Instruction: jump (or branch) directly to the label without satisfying any condition
Same meaning as (using C/Java): goto label
Technically, it’s the same as:
beq $0,$0,label
since it always satisfies the condition.

63. Compiling C/Java if into MIPS
Compile by hand
if (i == j) f=g+h; else f=g-h;
Use this mapping: f: $s0 g: $s1 h: $s2 i: $s3 j: $s4
Exit
i == j?
f=g+h
f=g-h
(false) i != j
(true) i == j

64. Compiling C/Java if into MIPS
Exit
i == j?
f=g+h
f=g-h
(false) i != j
(true) i == j
Compile by hand
if (i == j) f=g+h; else f=g-h;
Final compiled MIPS code:
beq $s3,$s4,True # branch i==j sub $s0,$s1,$s2 # f=g-h(false) j Fin # goto Fin True: add $s0,$s1,$s2 # f=g+h (true) Fin:
Note: Compiler automatically creates labels to handle decisions (branches). Generally not found in HLL code.

65. “And in Conclusion…”
Memory is byte-addressable, but lw and sw access one word at a time.
A pointer (used by lw and sw) is just a memory address, so we can add to it or subtract from it (using offset).
A Decision allows us to decide what to execute at run-time rather than compile-time.
C/Java decisions are made using conditional statements within if, while, do while, for.
MIPS Decision making instructions are the conditional branches: beq and bne.
New Instructions:
lw, sw, beq, bne, j

66. Green Card: OPCODES, BASE CONVERSION, ASCII (3)f 15
floor.w.f
sync
lui 24 2
Hexa-deci-mal 36
Deci-mal
STX
mul.f
srl j $
cvt.w.f
and
lbu
NUL
add.f
sll
(1)
ASCII
Binary
(2) MIPS funct (5:0)
(1) MIPS funct (5:0)
MIPS opcode (31:26)
(1) opcode(31:26) == 0
(2) opcode(31:26) == 17 ten (11 hex );
if fmt(25:21)==16 ten (10 hex ) f = s (single);
if fmt(25:21)==17 ten (11 hex ) f = d (double)
Note: 3-in-1 - Opcodes, base conversion, ASCII!

67. Green Card
green card /n./ [after the "IBM System/360 Reference Data" card] A summary of an assembly language, even if the color is not green. For example, "I'll go get my green card so I can check the addressing mode for that instruction."
Image from Dave's Green Card Collection: