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Languages and the Machine

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  1. Languages and the Machine Chapter 5 CS221

  2. Topics • The Compilation Process • The Assembly Process • Linking and Loading • Macros • We will skip • Case Study: Extensions to the Instruction Set – The Intel MMX™ and Motorola AltiVec™ SIMD Instructions

  3. Compilation Process • Assembly to Machine code fairly straightforward, but compilation is not • Translate a program written in a high level language into a functionally equivalent program in assembly language • Consider a simple high-level language assignment statement: • Foo = Bar + Zot + 15; • • Steps involved in compiling this statement into assembly code: • Lexical analysis: separate into tokens, Foo, =, +, etc. • Syntactic Analysis / Parsing : Determine that we are performing an assignment, VAR = EXPRESSION • Semantic Analysis : Determine that Foo, Bar, Zot are names, 4 is an integer • Code Generation : Determine the proper assembly code to perform the action • ld [Bar], %r0, %r1 • ld [Zot], %r0, %r2 • addcc %r1, %r2, %r1 • addcc %r1, 15, %r2 • st %r2, %r0, [Foo]

  4. Compiler Issues • Each compiler specific to a particular ISA • E.g., an int on one machine may be 32 bits, on another may be 64 bits • Cause of error in networking library ported to Alpha • Int issue not a problem in Java; JVM specifies 32 bits • E.g., in previous example, if the ISA allowed operands of addcc to be memory addresses, we could have done • addcc [Bar], [Zot], %r1 • addcc %r1, 15, [Foo] • Hopefully the compiler generates efficient code but optimization is a tough issue! • Cross compiler: one that generates code for a different ISA (example, CodeWarrior)

  5. Mapping Variables to Memory • Global variables • Accessible from anywhere in the program, given a fixed address • E.g., global variable X at memory address 400 • Local variables • Also called automatic variables • Defined inside a function or method, e.g. void foo() { int a,b; … } • These variables created when foo is invoked, destroyed when foo exits • These variables are created by pushing them on the stack when the function is invoked, and are popped off when the function exits

  6. Local Variables and the Stack • Recall that the stack typically grows downward in memory • Here we start with 1234 stored on the top of the stack Mem Mem 0 4 8 … 0 4 8 … FFFF 1234 1234 Push FFFF SP = 8 SP = 4

  7. Local Variables and the Stack • In our case, local variables are “pushed” on the stack upon entering the function • void foo() { int a; } • Copy SP into Frame Pointer FP (also called the Base Pointer, or BP) Mem before Foo Mem in Foo 0 4 8 … 0 4 8 … Var a 1234 1234 SP = 8 SP = 4 FP = 8

  8. Accessing Stack Variables • These variables are referenced as offsets from the frame pointer, called based addressing • To access a: [%fp – 4] Mem in Foo 0 4 8 … Var a Why not use [%sp] ? Consider pushing lots of stuff on the stack… Or data structures 1234 SP = 4 FP = 8

  9. C to ASM Example on x86 pushl %ebp movl %esp, %ebp subl $8, %esp movl $3, -4(%ebp) movl $4, -8(%ebp) movl -4(%ebp),%eax imul1 -8(%ebp),%eax movl %eax, c … .comm c,4,4 #include <stdio.h> int c; int main() { int a,b; a=3; b=4; c=a*b; }

  10. Arrays in Memory • Arrays may be allocated on the stack or allocated off the heap, a pool of memory where portions may be dynamically allocated. Access elements of an array a bit different than regular variables. • int A[10]; Array of 10 integers Mem allocated for A 0 4 8 … 40 A[0] A[1] … A[9] A (Base) = 4 ElementAddr = A + (Index*Size) e.g. A[2] is at 4 + (2*4) = 12

  11. If-Statements • Conditional statements map to a comparison and a branch instruction • C • if (x==y) statement1; else statement2; • Assembly (assume X in r1, Y in r2) • subcc %r1, %r2 ! Zero flag set if res=0 • bne Statement2 ! Branch if zero flag is not set • ! Statement1 code • ba StatementNext ! Branch always • Statement2: ! Statement2 code • StatementNext:

  12. Loops • While, Do-While, For loops implemented using the same conditional check and branch as the if-then statement • The branch returns back to previous code instead of jumping forward over code

  13. Production Level Assemblers • Allow programmer to specify location of data and code • Provide mnemonics for all instructions and addressing modes • Permit the use of symbolic labels to represent addresses and constants • Provide a means to specify the starting address of the program • Include a way to share variables between different assembled programs • Support macros

  14. Assembly Example

  15. Assembled Code

  16. Two Pass Assemblers • Most assemblers are “two-pass” • First pass • Determine addresses of all data and instructions • Perform any assembly-time arithmetic • Put definitions and constants into the symbol table • Second pass • Generate machine code • Insert actual addresses and values of symbols which are known from the symbol table • Two passes useful for forward references, i.e. referencing later on in the program

  17. Forward Reference

  18. Symbol Table • Generated during the first pass • Maps identifiers to values, table filled in as values are encountered and the program is parsed from top to bottom • .org 2048 ; Says assemble code starting at 2048 • const .equ value ; Defines const equal to value

  19. Assembled Program

  20. Final Tasks of the Assembler • Linking and Loading • We need the following additional info • Module name and size • Address start symbol • Information about global and external symbols • Information about any library routines • Values of constants • Relocation information

  21. Location of Programs in Memory • We have been using .org to specify a fixed start location • Typically we will want programs capable of running in arbitrary locations • If we are concatenating together different modules, the addresses for identifiers in the different modules must be relocated • Linker : software that combines separately assembled modules • Loader : software that loads another program into memory and may modify addresses if the program is loaded in a location different from the origin • Must also set appropriate registers, e.g. %SP

  22. Linking: .global and .extern • A .global is used in the module that a symbols is defined and .extern is used in every other module that refers to it

  23. Linking and Loading • Symbol tables for previous example • Symbols whose address might change market relocatable (not all addresses! Some may be fixed)

  24. DLL’s • Windows uses Dynamic Link Libraries, or DLL’s • Linking a common routine in many programs results in duplicate code from that common routine in each program • In a DLL, commonly used routines (e.g. memory management, graphics) present in only one place, the DLL • Smaller program sizes, each program does not need to have its own copy • All programs share the exact same code while executing • Don’t need recompiling or relinking • Disadvantages • Deletion of a shared DLL by mistake can cause problems • Versions must be the same • DLL code file can live in many places in Windows • “DLL Hell”

  25. Macros • An assembly macro looks kind of like defining a subroutine • For example, there say that there is no PUSH instruction to push data on the stack. We can make a macro for push:

  26. Macro Expansion • Given the previous macro, we could now write the following code: push %r15 ! Push r15 on the stack push %r20 ! Push r20 on the stack • Upon assembly, these macros are expanded to generate the following actual code: addcc %r14, -4, %r14 st %r15, %r14 addcc %r14, -4, %r14 st %r20, %r14

  27. Macros vs. Subroutines • Later we will see how to write actual subroutines we can call • Only one copy of the shared code in a subroutine • Tradeoffs • Subroutines • Takes up less memory since only one copy of the code • But slower than macros; subroutines have overhead of invoking and returning • Macros • Take up more space than subroutine call due to macro expansion for each occurrence of the macro • Faster than subroutines; no overhead to invoke/return

  28. Skipping for now • Discussion on Pentium MMX • We may return to this later if time permits