1 CS/COE0447 Computer Organization & Assembly Language Chapter 2 Part 3

2 Topics Register Usage Conventions Stack and Frame Pointers Procedure Calls Creating executables –Assembler, linker, loader –Compilers versus interpreters What does the assembler do for you?

3 Register Usage Convention NAMEREG. NUMBERUSAGE $zero0zero $at1reserved for assembler usage $v0~$v12~3values for results and expression eval. $a0~$a34~7arguments $t0~$t78~15temporaries $s0~$s716~23temporaries, saved $t8~$t924~25more temporaries $k0~$k126~27reserved for OS kernel $gp28global pointer $sp29stack pointer $fp30frame pointer $ra31return address

4 Stack and Frame Pointers Stack pointer ($sp) –Keeps the address to the top of the stack –$29 is reserved for this purpose –Stack grows from high address to low –Typical stack operations are push/pop Procedure frame –Contains saved registers and local variables –“Activation record” Frame pointer ($fp) –Points to the first word of a frame –Offers a stable reference pointer –$30 is reserved for this –Some compilers don’t use $fp

5 Procedure Calls Argument passing –First 4 arguments are passed through $a0~$a3 –More arguments are passed through stack Result passing –First 2 results are passed through $v0~$v1 –More results can be passed through stack Stack manipulations can be tricky and error-prone A programming assignment will involve the stack

6 “C Program” Down to “Numbers” swap: muli$2, $5, 4 add$2, $4, $2 lw$15, 0($2) lw$16, 4($2) sw$16, 0($2) sw$15, 4($2) jr$31 void swap(int v[], int k) { int temp; temp = v[k]; v[k] = v[k+1]; v[k+1] = temp; } … … … … … … … compiler assembler

7 To Produce an Executable source file.asm/.s source file.asm/.s source file.asm/.s object file.obj/.o object file.obj/.o object file.obj/.o library.lib/.a executable.exe assembler linker

8 Linker

9 An Object File Header –Size and position of other pieces of the file Text segment –Machine codes Data segment –Binary representation of the data in the source Relocation information –Identifies instructions and data words that depend on absolute addresses Symbol table –Keeps addresses of global labels –Lists unresolved references Debugging information –Contains a concise description of the way in which the program was compiled

10 Interpreter vs. Compiler InterpreterCompiler Concept Line-by-line translation and execution Whole program translation and native execution Example Java, BASIC, …C, Fortran, … + Interactive Portable (e.g., Java Virtual Machine) High performance – Low performance Binary not portable to other machines or generations

11 An Assembler Expands macros –Macro is a sequence of operations conveniently defined by a user –A single macro can expand to many instructions Determines addresses and translates source into binary numbers –Record in “symbol table” addresses of labels –Resolve branch targets and complete branch instructions’ encoding –Record instructions that need be fixed after linkage Packs everything in an object file “Two-pass assembler” –To handle forward references

12 Assembler Directives Guides the assembler to properly handle following codes with certain considerations.text –Tells assembler that codes follow.data –Tells assembler that data follow.align –Directs aligning the following items.global –Tells to treat the following symbol as global.asciiz –Tells to handle the following as a “string”

13 Macro Example.data int_str:.asciiz“%d”.text.macroprint_int($arg) la $a0, int_str mov $a1, $arg jal printf.end_macro … print_int($7) la $a0, int_str mov $a1, $7 jal printf To use macros in MARS, need to run it from the command line

14 High-Level Lang. vs. Assembly High-level languageAssembly Example C, Fortran, Java, …- + High productivity – Short description & readability Portability Low productivity – Long description & low readability Not portable – Limited optimization capability in certain cases With proper knowledge and experiences, fully optimized codes can be written

15 Branch and Jump Instructions Review

16 Instruction Format beq, bne PC = PC BranchAddr Without delayed branching (in MARS!!) –PC = PC + BranchAddr BranchAddr = {14{immediate[15]},immediate,2'b0} Two issues: –Assembly  machine code –Execution of the machine code 16-bit immediatertrsop I

17 bne $t0,$s5,exitloop addi $s3,$s3,1 j loop exitloop: add $s6,$s3,$zero 0x x x c 0x BNE machine code in binary: BNE machine code in hex: When BNE instruction is executed (condition true): Next address = PC + BranchAddr Next address = c = … address of the exitloop inst! BranchAddr = {14{immediate[15]},immediate,2'b0} = { , ,00} = = 0x c

18 BranchAddr: Why 2’b0 at the end? BranchAddr might have been: {number,00} = number * 4 (like shifting left by 2) Recall: the immediate field of the instruction contains the number of instructions away the label is Each instruction is 4 bytes, so multiplying by 4 gives you the number of bytes away it is. This is the number to add to the PC to jump to the label. {16{immediate[15]},immediate}

19 BranchAddress: Why 2’b0 at the end? If immediate instead were the number of bytes away the label is, then we would be wasting 2 bits Since all instruction addresses are multiples of 4, the bottom 2 bits are always 00 By not including those bits in the immediate field of the machine code, branch instructions can be used to jump 4 times further away

20 Nice question for an Exam What would change in the previous example, if delayed branching were used in MARS? The immediate value would be 1 less With delayed branching: –PC = PC BranchAddress –The “4” is due to the fact that the PC has already been incremented to point to the next instruction In EG (slide 17): imm would be PC = PC BranchAddress BranchAddress = {14{0}, ,00} = 0x PC = 0x = 0x

21 Instruction Format, cont’d The address of next instruction is obtained by concatenating with PC PC = {PC[31:28],address,2’b0} 26-bit addressopJump

22 0x bne $s4, $s5, ELSE 0x c add $s3, $s2, $s5 0x j EXIT ELSE: 0x sub $s3, $s2, $s5 0x addi $s5, $s5, 1 0x c EXIT: addi $s4,$s4,1 j instruction machine code: Hex: b Look at execution: PC = {PC[31:28],address,00} PC[31:28] = 0000 address = {0000, address, 00} = BIN c HEX The address EXIT stands for!