Computing Systems Instructions: language of the computer

2 Instructions  Instructions are the language of the machine  We’ll be work with the MIPS instruction set architecture  Design goals  find a language that make it easy to build the hardware and the compiler  maximizing performance and minimizing cost  Stored-program concept  programs (= instructions and data) can be stored in memory as numbers  Fetch & execute cycles  Fetch the instruction from memory and put it into a special register (IR)  The bits in the register “control” the subsequent actions required to execute the task specified by the instruction continue with “next” instruction

3 Instruction set characteristic  Small  Few primitive instructions  Simple  Few instruction formats  Regular  Instructions can be handled in similar ways  Complete  support all kind of high level language instructions  Efficient  High level language instructions can be mapped efficiently  Compatible

4 Definition of the architecture  Data types - bit, byte, word, unsigned integer, char, …  Operations - arithmetic, logical, shift, flow control, data transfers, …  # of operands (3, 2, 1, or 0 operands) - number of operands affect the instruction length  Registers  Memory organization

5 MIPS arithmetic  all arithmetic instructions have 3 operands  operands order is fixed (destination first) Design principle: Simplicity favors regularity. Why ?  of course this complicate some things … C codeMIPS code c = a+badd $s0, $s1, $s2 C codeMIPS code f = (g + h) – (i + j) add $t0, $s1, $s2 add $t1, $s3, $s4 sub $s0, $t0, $t1

6 Registers vs. memory  Arithmetic operands must be registers, - only 32 registers provided - each register is 32 bits (word) Design Principle: Smaller is faster. Why ?  Compiler associates variables with registers, but …  What about programs with lots of variable or complex data structures ?  Only a small amount of data can be kept in the computer’s registers, the rest is kept in memory  We’ll need instructions that transfer data between memory and registers Absolute truth ? ProcessorI/O Control Datapath Memory Input Output

7 Memory organization  Viewed as a large, single-dimension array, with an address.  A memory address is an index into the array  "Byte addressing" means that the index points to a byte of memory bits of data

8 Memory organization  Bytes are nice, but most data items use larger "words"  For MIPS, a memory word is 32 bits wide (= 4 bytes)  2 32 bytes with byte addresses from 0 to  2 30 words with byte addresses 0, 4, 8, – 4  Words are aligned (alignment restriction)  words start at addresses that are multiple of 4  what are the least 2 significant bits of a word address?

9 Bytes order within a word  Which byte is first and which is last ?  There are two choices  Least significant byte is the at rightmost end (= little end)  Least significant byte is the at leftmost end (= big end)  MIPS uses Big Endian Big Endian Little Endian

10 MIPS data transfer instructions  load (it copies data from memory to a register)  store (copies data from a register to memory)  Memory address for load and store has two parts  A register whose contents is known (called base register or index register)  An offset to be added to the base register content  This way the address is 32 bits  The offset can be positive or negative (2’s complement) C codeMIPS codeMeaning A[12] = h + A[8] lw $t0, 32($s3) add $t0, $s2, $t0 sw $t0, 48($s3) $t0 = Memory[$s3 + 32] $t0 = $s2 + $t0 Memory[$s3 + 48] = $t0

11 Machine language  MIPS assembly instructions are translated into machine instructions (“binary numbers”)  Instructions, like registers and words of data, are also 32 bits long  Example: add $t1, $s1, $s2  registers are represented as numbers: $t1=9, $s1=17, $s2=18, …  The format above is called R-format (for register)  Can you guess what the field names stand for? 6 bits5 bits 6 bits

12 Machine language  Consider the load-word and store-word instructions  We must specify two registers and a constant (=immediate operand)  Regularity principle would suggest to use for the constant one of the 5-bit fields  Problem: the constant would be limited to only 32 !!!)  Solution: we introduce a new format called I-type (for immediate)  The 16 bit number is in 2’s complement form Design principle: good design demands good compromises 6 bits5 bits 16 bits

13 Examples $t0 (rt) $s3 (rs) sw (op) lw (rs) $s2 (rt) $t0

14 MIPS logical operations The above instructions are all R-type Logical operationExampleMeaning shift left logical sll $s1, $s2, 10$s1 = $s2 << 10 shift right logical srl $s1, $s2, 10$s1 = $s2 >> 10 bit-by-bit and and $s1, $s2, $s3$s1 = $s2 & $s3 bit-by-bit oror $s1, $s2, $s3$s1 = $s2 | $s3 bit-by-bit nornor $s1, $s2, $s3$s1 = ~ ($s2 | $s3)

15 Example  sll $s1, $s2, 10  srl $s1, $s3, oprtrdshamtfunc oprtrdshamtfunc

16 Control flow  Decision making instructions  alter the program control flow,  i.e., change the "next" instruction to be executed  MIPS conditional branch instructions:  bne $t0, $t1, Label  beq $t0, $t1, Label C codeMIPS code if (i==j) h=i+j; bne $s3, $s4, Label add $s5, $s3, $s4 Label: …

17 Control flow  MIPS unconditional branch instruction (jump)  j Label Can you build a simple for loop? C codeMIPS code if (i==j) f=g+h; else f=g-h bne $t0, $t1, Else add $s5, $s3, $s4 j Exit Else: sub $s5, $s3, $s4 Exit: …

18 Control flow  We have: beq (test for equality) and bne (test for inequality)  But, sometime is useful to see if a variable is less than another one (branch-less-than)  MIPS provides the instruction set-on-less-than (slt)  The format is R-type MeaningMIPS code if (s1<s2) t0=1; else t0=0 slt $t0, $s1, $s2

19 Control flow  Case switch statement  can be implemented as a chain of if-then-else statements  or more efficiently as a table of addresses  MIPS provides a jump register instruction (jr)  It is an unconditional jump to the address specified in a register  Example: jr $s0

20 Control flow – Instruction formats  slt and jr are R-format oprsrtrdshamtfunc slt $s1, $s2, $s oprsfunc jr $s1

21 Control flow – Instruction formats  beq and bne are I-format instructions  The 16-bit number is in 2’s complement form  The 16-bit number specifies the # of instructions to be skipped  Most branches are local [Principle of locality]  Next memory address = PC + (16-bit number x 4) oprsrt16-bit number beq $s1, $s2, 100

22 Control flow – Instruction formats  The unconditional jump instruction requires a new instruction format (J-format)  Example: j  The address of the next instruction is obtained by concatenating the 4 upper bits of the PC with the 26-bit address shifted left 2 bits  Next memory address = {PC[31:28], INS[25:0], 2’b00}  Address boundaries of 256 MB = (2 28 ) op26-bit number

23 MIPS Registers convention Register 1 ($at) reserved for assembler, ($k0-$k1) for operating system

24 Constants  Small constant operands occur quite frequently in programs (50% of operands)  By including constants inside instructions, execution become much faster than if constants were loaded from memory Design Principle: make the common case fast w/o immediate instructionwith immediate instruction lw $t0, AddrConstant4($s1) add $s3, $s3, $t0 addi $s3,$s3,4

25 Constants  Why doesn’t MIPS have a subtract immediate operation ?  Constants are frequently short and fit into the 16-bit field  But, what if they are bigger ?  It would be convenient to have a 32-bit constant or address !!!! CategoryInstructionExampleMeaning Arithmeticadd immediateaddi $s1, $s2, 100$s1 = $s Logical and immediateandi $s1, $s2,100$s1 = $s2 & 100 or immediateori $s1, $s2, 100$s1 = $s2 | 100 Conditional branch set less than immediate slti $s1, $s2, 100 If ($s2<100) $s1=1 else $s1=0

26 How about large constants ?  The compiler or the assembler must break large constants into pieces and then reassemble them into registers  We'd like to be able to load a 32 bit constant into a register  Must use two instructions, new "load upper immediate" instruction  Then must get the lower order bits right, i.e., ori $t0, $t0, filled with zeros ori lui $t0,

27 Assembly Language vs. Machine Language  Assembly provides convenient symbolic representation  much easier than writing down numbers  e.g., destination first  Machine language is the underlying reality  e.g., destination is no longer first  Assembly can provide 'pseudoinstructions'  e.g., “move $t0, $t1” exists only in assembly  would be implemented using “add $t0,$t1,$zero”  When considering performance you should count real instructions

28 Translating and starting a program

29 Overview of MIPS  simple instructions all 32 bits wide  very structured  only three instruction formats  rely on compiler to achieve performance op rs rt rdshamtfunct op 26 bit number RIJRIJ op rs rt 16 bit number

30 Summary – MIPS operands

31 Summary – MIPS instructions

32 Addressing modes in MIPS  Register addressing (e.g., add, sll, nor, slt, …)  the operand is a register  Immediate addressing (e.g., addi, ori, …)  the operand is a constant within the instruction  Base addressing (e.g. lw, sw) [register+offset]  The operand is at the memory location whose address is the sum of a register and a constant in the instruction  PC-relative addressing (e.g., bne, beq) [PC + offset]  The address is the sum of the PC and a constant in the instruction  Pseudo-direct addressing (e.g., j) [PC concatenation]  The address is the concatenation of a value in the instruction with the upper bits of the PC

33 Addressing modes in MIPS

34 Alternative architectures  Design alternative:  provide more powerful operations  goal is to reduce number of instructions executed  danger is a slower cycle time and/or a higher CPI  RISC vs. CISC  virtually all instruction sets since 1982 are RISC  common architectures: 80x86, IA-32, PowerPC, …

35 “Spilling” registers  Many programs have more variables than computers have registers  The compiler tries to keep the most frequently used variables in registers and places the rest in memory  The process of putting less commonly used variables (or those needed later) into memory is called spilling registers

36 Supporting procedures  Goal:  structure programs so that  it is easier to understand and reuse code  Execution of a procedure 1. place parameters in a place where the procedure can access them 2. transfer control to the procedure 3. acquire the storage resources needed for the procedure 4. Place the result value in a place where the calling program can access it 5. Return control to the point of origin  Basic MIPS facilities  $a0-$a3  4 argument registers in which to pass parameters  $v0-$v1  2 value registers in which to return values  $ra  return address register to return to the point of origin  jal ProcedureAddress  jump-and-link (it saves PC+4 in $ra)  jr $ra  jump register

37 What if we need more registers ?  A compiler may need more registers for a procedure than the four argument and two return value registers  The local variables we need may not even fit in the MIPS registers  also any register needed by the caller must be restored to the values they contained before the procedure was invoked  we need to spill the registers to memory  The ideal data structure for spilling registers is a stack (last in first out)  push – place data on the stack  pop – remove data from the stack

38 The stack  MIPS allocate a register just for the stack: the stack pointer ($sp)  it points to the most recently allocated address in the stack  Stacks grow from higher addresses to lower addresses  push values on the stack by subtracting from the $sp  pop values from the stack by adding to the $sp

39 Example – compiling a leaf procedure int leaf_example (int g, int h, int i, int j) { int f; f = (g+h) – (i+j); return f; } … leaf_example main … … jal leaf_example … Memory

40 Example – compiling a leaf procedure leaf_example: addi $sp,$sp,-12 # adjust stack to make room for 3 items sw $t1, 8($sp) # save $t1 for later us by the caller sw $t0, 4($sp) # save $t0 for later us by the caller sw $s0, 0($sp) # save $s0 for later us by the caller add $t0,$a0,$a1 # t0 = g+h add $t1,$a2,$a3 # t1 = i+j sub $s0,$t0,$t1 # s0 = t0–t1 = (g+h)-(i+j) add $v0,$s0,$zer0 # v0 = s0+0 (return f) lw $s0, 0($sp) # restore $s0 for caller lw $t0, 4($sp) # restore $t0 for caller lw $t1, 8($sp) # restore $t1 for caller addi $sp,$sp,12 # adjust stack to delete 3 items jr $ra # jump back to calling routine

41 Reduce register spilling  To avoid saving and restoring a register whose value is never used, MIPS separate temporary registers from saved registers:  $t0-$t9: are not preserved by the callee (called procedure)  $s0-$s7: must be preserved on a procedure call (if used, the callee saves and restore them)  Thus, the assembly code generated for leaf_example by a compiler can be more compact !!!  Can you guess how the code should look like ?  When a compiler finds a leaf procedure it exhaust all temporary registers before using registers it must save

42 Concluding Remarks  Instruction complexity is only one variable  lower instruction count vs. higher CPI / lower clock rate  Design Principles:  simplicity favors regularity  smaller is faster  good design demands compromise  make the common case fast  Instruction set architecture  a very important abstraction !

43 Common misconceptions and mistakes  More powerful instructions mean higher performance  Write in assembly language give the highest performance  Forgetting that sequential word addresses in machines with byte addressing do not differ by one  Using a pointer to an automatic variable outside its defining procedure