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Abstractions and Computers and the MAL programming Language

Abstractions and Computers and the MAL programming Language. Computer Architecture. Definition: Interface between a computers hardware and its software. Defines exactly what the computer’s instructions do, and how they are specified. Instruction Set Architecture (ISA). Computer Architecture.

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Abstractions and Computers and the MAL programming Language

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  1. Abstractions and Computers and the MAL programming Language 1

  2. Computer Architecture Definition: Interface between a computers hardware and its software. Defines exactly what the computer’s instructions do, and how they are specified.Instruction Set Architecture (ISA) 2

  3. Computer Architecture • SAL – Simple Abstract Language • MAL – MIPS Assembly Language • TAL – True Assemble Language MIPSmachine language SAL MAL TAL 3

  4. Computer Architecture High Level Language Assembly Language Machine Language Compiler Assembler Compiler: A computer program that translates code written in a high level language into an intermediate level abstract language. 4

  5. Computer Science Definition: Fundamentally the study of algorithms and data structures. Abstraction: Use of level of abstraction in software design allows the programmer to focus on a critical set of problems without having to deal with irrelevant details. 5

  6. int average (a, b) begin avg = (a+b)/2; return (avg); end main () … x = 4; y = 2; k = average (x,y); print (“%d”, k); … Computer Science Procedure or Function 6

  7. Computer CPU(MIPS) Memory Control info Write data Read data • CPU Interacts with the memory in 3 ways: • fetches instructions • loads the value of a variable • stores the new value of a variable • Memory is capable of only 2 operations: • reads – a load or a fetch • writes – operation of a storing the value of a variable 7

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  9. Instruction Fetch / Execute Cycle In addition to input & output a program also: • Evaluates arithmetic & logical functions to determine values to assign to variable. • Determines the order of execution of the statements in the program. In assembly this distinction is captured in the notion of Arithmetic, logical, and control instructions. 9

  10. Instruction Fetch / Execute Cycle Arithmetic and logical instructions evaluate variables and assign new values to variables. Control instructions test or compare values of a variable and makes decisions about what instruction is to be executed next. Program Counter (PC) Basically the address at which the current executing instruction exists. 10

  11. Instruction Fetch / Execute Cycle • load rega, 10 • load regb, 20 • add regc, rega, regb • beq regc, regd, 8 • store regd, rege • store regc, regd • load regb, 15 • load rega, 30 PC Address 11

  12. Instruction Fetch / Execute Cycle The CPU begins the execution of an instruction by supplying the value of the PC to the memory & initiating a read operation (fetch). The CPU “decodes” the instruction by identifying the opcode and the operands. PC increments automatically unless a control instruction is used. 12

  13. Instruction Fetch / Execute Cycle • For example: • PC  add A, B, C • CPU fetches instruction • Decodes it and sees it is an add operation, needs to get values for the variables “B” & “C” • CPU executes a load operation, gives address of variable “B” • Does the same for variable “C” • Does the “add” operation and stores the result in location of variable “A” 13

  14. Instruction Fetch / Execute Cycle Branch – like a goto instruction, next instruction to be fetched & executed is an instruction other than the next in memory. add A, B, C beq A, 5, fred sub A, D, 3fred: sub A, D, 4 If A==5 then next instruction is at fred 14

  15. Breaking down an instruction add a, b, c add a b c Source registers Opcode Destination register 15

  16. Locality of reference We need techniques to reduce the instruction size. From observation of programs we see that a small and predictable set of variables tend to be referenced much more often than other variables. 16

  17. Basically, locality is an indication that memory is not referenced randomly. This is where the use of registers comes into play. 17

  18. Registers and MAL IO ALU RegisterArray Memory(disk) Memory Ctrl Data cache Inst. cache Program “code” is in memory (or cache), use registers to hold commonly used variables for faster execution 18

  19. CISC vs. RISC CISC : complex instruction set computerLots of instructions of variable size, very memory optimal, typically less registers. RISC : reduced instruction set computer Less instructions all of a fixed size, more registers, optimized for speed. Usually a “Load/Store” architecture. 19

  20. Specifying addresses For a load/store architecture, registers are used to supply source operands and receive results from all instructions except loads and stores. Basically, load the registers with the operands from memory first, then perform the operation. 20

  21. How do we fit the “stuff” in 32-bit instructions? So we have arithmetic instructions and branch type instructions that cannot contain all the needed info in a single 32-bit word. 21

  22. Ways to get an effective address 1. Instruction might occupy 2 words. Effectiveaddress opcode reg address 2. Instruction might specify a register that contains the address, 1 word instruction. reg Effectiveaddress opcode reg address 22

  23. Effective Address Calculation 3. Instruction might specify a small constant and a second register, 1 word instruction. opcode reg constant address + Effective address reg 23

  24. Effective Address Calculation 4. The instruction might specify 2 additional registers, 1 word instruction. opcode reg reg reg address address reg + Effective address 24

  25. Addressing modes Methods a computer uses to specify an address within an instruction. 25

  26. Addressing Modes • ImmediateThe operand is contained directly in the instruction. Ex: li reg1, 5 • RegisterThe operand is contained in a register. Ex: add reg1, reg2, reg3 • DirectThe address of the operand is contained in the instruction (two-word instruction) 26

  27. Addressing Modes • Register DirectThe address of the operand is contained in a register. Ex: lw reg1, reg3 • Base DisplacementThe address is computed as the sum of the contents of a register (the base) and a constant contained in the instruction (the displacement). Ex: lw reg1, 5(reg3) 27

  28. Addressing Modes • IndirectThe instruction specifies a register containing an address the content of which is the address of the operand opcode reg reg address Memoryaddress address Effective address 28

  29. MAL (MIPS Assembly Language) MIPS is a load/store RISC architecture • 2 distinct register files: • 32 general registers • 16 floating point registers 29

  30. MAL The 32 general registers are numbered $0 - $31. $0 is always the value “Zero”.$1 is used by the assembler.$26 & $27 are used by the operating system.$28, $29, & $31 have special conventions for the use of them. 30

  31. Common aliases for registers $2-$3 $v0-$v1 procedure results $4-$7 $a0-$a3 parameters for procedure $8-$15 $t0-$t7 temporary registers $24-$25 $t8-$t9 $16-$23 $s0-$s7 saved registers $30 $s8 $29 $sp stack pointer $31 $ra return address register 31

  32. MAL The 16 floating point registers are intended exclusively for holding floating point operands. These registers are 64-bits in size for holding both single precision (32-bit) floats and double precision (64-bit) floats. These registers are named $f0, $f2, $f4,…., $f30. Why? 32

  33. MAL MAL uses a single, versatile addressing mode for its regular load store . Base displacement. General since its special cases provide for both direct and register direct address. 33

  34. MAL Syntax Syntax of MAL one instruction, declaration per line comments are anything on a line following # comments may not span lines MAL has 3 basic types: Integer Floating point Character 34

  35. MAL Syntax “C” type variablename; “MAL” variablename: type value type is .word (integer) .byte (character) .float (floating point) value is optional – the initial value 35

  36. MAL Syntax Examples: flag: .word 0 counter: .word 0 variable3: .word e: .float 2.71828 uservalue: .byte letter: .byte ‘a’ • One declaration per line • Default initial value is 0(but you may lose points if you depend on this) 36

  37. MAL Syntax Directives give information to the assembler. All directives start with ‘.’ (period) Examples: .byte .word .float main: # tells simulator to start execution at this location. .data# .data identifies the start of the declaration section # there can be more than 1 .data sections in a program. 37

  38. MAL Syntax .text # identifies where instructions are, there can be # more than 1 .text sections in a program .asciiz “a string.\n” # places a string into memory and null terminates # the string .ascii “new string.”# places a string into memory with no null # termination. 38

  39. MAL lw $s1, x lw $s2, y move $s3, $s2 add $s3, $s1, $s2 sub $s3, $s1, $s2 mul $s3, $s1, $s2 div $s3, $s1, $s2 rem $s3, $s1, $s2 sw $s3, z MAL Syntax C z = y; z = x + y; z = x - y; z = x * y; z = x / y; z = x % y; An immediate is a value specified in an instruction, not in the .data section. Examples: li $s2, 0 # load immediate add $s2, $s2, 3 # add immediate 39

  40. MAL Program Simple MAL program .data avg: .word 0 i1: .word 20 i2: .word 13 i3: .word 2 .text main: lw $s1, i1 lw $s2, i2 lw $s3, i3 add $s4, $s1, $s2 div $s4, $s4, $s3 sw $s4, avg li $2, 10 # done cmd syscall 40

  41. Program Execution • Assembler translates to executable – machine language • Linker combines multiple MAL files – if any • Loader puts executable into memory and makes the CPU jump to first instruction or “main:” • Executes • When executing is done returns control to OS • Or simulator or monitor • Load again to run again with different data • In this case, assemble again, too, since data is in program. 41

  42. MAL Programming HLL – if/else statements… if (condition) statement; else statement; “C” if (count < 0) count = count + 1; 42

  43. MAL Programming “MAL” lw $t1, count bltz $t1, ifstuff b endif ifstuff: add $t1, $t1, 1 endif: # next instruction goes here “OR” lw $t1, count bgez $t1, endif add $t1, $t1, 1 endif: # next instruction goes here 43

  44. MAL Programming Loops can be built out of IF’s – WHILE: “C” while (count > 0) { a = a % count; count--; } 44

  45. MAL Programming “MAL” lw $s1, count lw $s2, a while: blez $s1, endwhile rem $s2, $s2, $s1 sub $s1, $s1, 1 b while endwhile: sw $s2, a sw $s1, count 45

  46. MAL Programming Repeat loops “C” /* do statement while expression is TRUE */ /* when expression is FALSE, exit loop */ do { if (a < b) a++; if (a > b) a--; } while (a != b) 46

  47. MAL Programming “MAL” lw $s3, a lw $s4, b repeat: bge $s3, $s4, secondif add $s3, $s3, 1 secondif: ble $s3, $s4, until sub $s3, $s3, 1 until: bne $s3, $s4, repeat 47

  48. MAL Programming While Loops (Part II) “C” while ( (count < limit) && (c ==d) ) { /* loop’s code goes here */} “MAL” while: ? ? # loop code goes here ? endwhile: 48

  49. MAL Programming For loops “C” for ( I = 3; I <= 8; I++) { a = a+I;} “MAL” ? for: ? ? ? ? endfor: 49

  50. Procedure Calls Simple procedure calls require 2 instructions: • “JAL” Jump and Link • Link means save the return address in $ra ($31) • “JR” Jump Register • Be careful with registers!! • Cannot nest unless $ra is saved elsewhere • Cannot be recursive without a stack 50

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