Do-While Example In C++ do { z--; while (a == b); z = b; In - PowerPoint PPT Presentation

Do-While Example • In C++  do {  z--;  while (a == b);  z = b; • In assembly language  loop: addi $s2, $s2, -1  beq $s0, $s1, loop  or $s2, $s1, $zero 25

Comparisons • Set on less than ( slt ) compares its source registers and sets its destination register to 1 if src1 < src2 and to 0 otherwise (branch-if-less-than)  Syntax: slt dest , src1 , src2 • Set on less than immediate ( slti ) perform the same comparison, but its second source operation is an immediate value  Syntax: slti dest , src1 , immed • Combined with beq and bne these instructions can implement all possible relational operators 26

Relational Branches (1) • Branch if less than  slt $t0, src1 , src2  bne $zero, $t0, label • Branch if greater than or equal  slt $t0, src1 , src2  beq $zero, $t0, label 27

Relational Branches (2) • Branch if greater than  slt $t0, src2 , src1  bne $zero, $t0, label • Branch if less than or equal  slt $t0, src2 , src1  beq $zero, $t0, label 28

Case/Switch Statement * • Can be implemented like a chain of if-then- else statements • Using a jump address table is faster • Must be able to jump to an address loaded from memory • Jump register ( jr ) gives us that ability • Syntax: jr src  Instruction: jr $t1 #go to address in $t1 29

Compiling a Case (Switch) Statement Memory switch (k) { case 0: h=i+j; break; /*k=0*/ L2 case 1: h=i+h; break; /*k=1*/ L1 case 2: h=i-j; break; /*k=2*/ $t4 → L0 • Assuming three sequential words in memory starting at the address in $t4 have the addresses of the labels L0, L1, and L2 and k is in $s2 add $t1, $s2, $s2 #$t1 = 2*k add $t1, $t1, $t1 #$t1 = 4*k add $t1, $t1, $t4 #$t1 = addr of JumpT[k] lw $t0, 0($t1) #$t0 = JumpT[k] jr $t0 #jump based on $t0 L0: add $s3, $s0, $s1 #k=0 so h=i+j j Exit L1: add $s3, $s0, $s3 #k=1 so h=i+h j Exit L2: sub $s3, $s0, $s1 #k=2 so h=i-j Exit: . . . 30

MIPS Organization Processor Memory Register File 1…1100 src1 addr src1 5 data 32 src2 addr 32 5 registers dst addr read/write ($zero - $ra) src2 5 addr write data data 2 30 32 32 32 words 32 bits br offset read data 32 Add PC 32 32 32 32 Add 32 4 write data 0…1100 Fetch 32 0…1000 PC = PC+4 32 4 5 6 7 0…0100 32 ALU 0 1 2 3 0…0000 Exec Decode 32 word address 32 bits 32 (binary) byte address (big Endian) 31

Variable Index into Array • Previous method for accessing array elements will only work if index is constant • If index is a variable we must compute the address in code • Formula: address + (index × width) • Since width is usually a power of two, we can use a left shift to perform the multiplication 32

Structures • Like an array, a structure (struct) is made up of several smaller data elements stored contiguously in memory • However the elements do not have to all be the same type • The compiler will construct a table mapping each structure member to an offset 33

Bit Fields • Structure members can be bit fields • In this case multiple members are packed into a single byte or word • To retrieve the value of a bit field, AND the word with a mask and then shift right • To set the value of a bit field use a left shift and an OR 34

Procedures * • A function (or procedure) is a sequence of instructions that can be used to perform a task • When a function is called control transfers to the function; once the function completes its task it returns • Functions may accept parameters (arguments) and/or return a value • A function can also define local variables for its use which exist only until the function returns • Functions can call other functions 35

Call and Return • Call and return are nothing more than unconditional jumps • The calling function (the caller ) jumps to the beginning of the function • The called function (the callee ) jumps back to the point from which it was called ( return address ) • Since a function can be called from any number of places the return address changes with each call 36

Jump and Link • The jump and link ( jal ) instruction performs an unconditional jump just like j , but first saves the address of the next instruction in the return address register ( $ra )  jal label • A function returns by jumping to the address stored in that register:  jr $ra 37

Example (1) • Calling a function:  jal function1 • Function:  function1: add $t0, $t1, $t2  sub $t3, $t4, $t5  jr $ra 38

Calling Conventions • We need a way to pass parameters to a function and get a return value from it • A calling convention is a set of rules that define how parameters and return values are passed to/from functions as well as which registers a function can use • Allows us to call functions compiled by different compilers and even languages 39

Parameters/Return Value • Most MIPS software uses the same calling convention • The first four parameters are placed in registers ($a0-$a3) additional parameters are placed on the stack • Return values go in registers $v0-$v1 40

MIPS Registers Name Number Usage Saved on Call? $zero 0 N/A Zero (hardware) $at 1 No reserved Assembler Temp $v0-$v1 2-3 Return Value/Temporaries No $a0-$a3 4-7 Arguments No $t0-$t9 8-15, 24-25 Temporaries No $s0-$s7 16-23 Saved Values Yes $k0-$k1 26-27 Reserved for OS Kernel No $gp 28 Global Pointer Yes $sp 29 Stack Pointer Yes $fp 30 Frame Pointer Yes $ra 31 Yes Return Address (hardware) 41

Example (2) • Function call  or $a0, $s0, $zero  jal function2  or $s0, $v0, $zero • Function definition  function2: add $v0, $a0, $a0  jr $ra 42

Register Use • Callee-saved registers are owned by caller; if a function uses a callee-saved register it must save the old value and restore that value before it returns • Caller-saved registers are owned by the callee; if a caller is using a caller-saved register, it must save the old value before calling the function and restore it after the function returns • In MIPS, $s0-$s7 are callee-saved and $t0-$t9 are caller-saved 43

The Stack • Data that a function needs to save in memory is pushed onto the stack • Before returning, the function pops the data off of the stack • In MIPS, the stack pointer ($sp) contains the address of the last word pushed onto the stack • The stack grows downward from higher addresses to lower ones 44

Spilling Registers • What if the callee needs more registers? What if the procedure is recursive?  uses a stack – a last-in-first-out queue – in memory for passing additional values or saving (recursive) return address(es)  One of the general registers, $sp, is used to address the stack (which “grows” from high address to low high addr address)  add data onto the stack – push top of stack $sp $sp = $sp – 4 data on stack at new $sp  remove data from the stack – pop data from stack at $sp low addr $sp = $sp + 4 45

Accessing the Stack • Push a word onto the stack  addi $sp, $sp, -4  sw src , 0($sp) • Pop a word off of the stack  lw dest , 0($sp)  addi $sp, $sp, 4 • Typically we batch multiple pushes and pops together 46

Accessing the Stack (cont'd) • Push three words onto the stack  addi $sp, $sp, -12  sw $t0, 8($sp)  sw $t1, 4($sp)  sw $s0, 0($sp) • Pop three words off of the stack  lw $s0, 0($sp)  lw $t1, 4($sp)  lw $t0, 8($sp)  addi $sp, $sp, 12 47

Stack Illustration High address $sp $sp Contents of register $t1 Contents of register $t0 $sp Contents of register $s0 Low address a. b. c. 48

Data on the Stack • Data a function puts on the stack makes up its stack frame or activation record • If there are more than four parameters, the extra parameters must be pushed onto the stack • Non-leaf functions must save the value of the return address register ($ra) 49

Data on the Stack (cont'd) • Any registers that must be saved are stored on the stack • Also local variables that won't fit into registers (either because there are no registers left or the data is too big) • Commonly a frame pointer ($fp) is used in addition to the stack pointer, since the stack pointer could change during the function 50

Stack Frame High address $fp $fp $sp $sp $fp Saved argument registers (if any) Saved return address Saved saved registers (if any) Local arrays and structures (if any) $sp Low address b. c. a. 51

Where Data is Stored • The static data area holds global variables  Fixed size  In MIPS, the global pointer ($gp) contains the address of this area • The stack holds local variables and function- related information  Dynamic and managed automatically by the compiler • The heap holds data allocated through new or malloc  Dynamic and managed by the programmer 52

Program Memory $sp 7fff ffff Stack hex Dynamic data $gp 1000 8000 Static data hex 1000 0000 hex Text pc 0040 0000 hex Reserved 0 53