Computer Structure - The Instruction Set (2) Goal: Implement Functions in Assembly  When executing a procedure (function in C) the program must follow the next 6 steps: 1. Place parameters in a place the procedure can access them. 2. Jump to the procedure code. 3. Allocate storage needed for the procedure. 4. Perform the task. 5. Place the result(s) in a place the calling program can access. 6. Return to the point of origin. 1/17

Computer Structure - The Instruction Set (2) Registers for Procedures  MIPS software allocates the following of its 32 registers for procedure calling:  $a0 - $a3 : 4 argument registers  $v0 - $v1 : 2 return value registers  $ra : return address register to return to point of origin.  2 new instructions that support procedures are:  jal ProcedureAddress Jump and link, jump to the procedure and save the address of the following instruction in $ra.  jr register Jump register, jump to the address in the register. 2/17

Computer Structure - The Instruction Set (2) Using More Registers  If the compiler needs more than the 4 input and 2 output registers, it can use other registers.  But it must restore the values in those registers to the values before the procedure was called.  This is a case of register spill, register values are saved in memory.  The ideal structure for saving registers is the stack, values are pushed onto the stack before the procedure call and poped out after.  A register called the stack pointer ($sp ) points to the address in memory where the stack resides. 3/17

Computer Structure - The Instruction Set (2) Compiling a Leaf Procedure int leaf(int g,int h,int i,int j){ int f;// the arguments are in $a0-$a3, f=(g+h) - (i+j); //f is in $s0 return f; } Before continuing we will introduce a new instruction: addi $t0,$t0,10# $t0=$t0+10 add immediate (addi) has as one of its operands an immediate value. addi is of the I-type instructions and in fact gives the format its name. 4/17

Computer Structure - The Instruction Set (2) The Assembly Version subi $sp,$sp,12#make room for 3 items on stack sw $t1,8($sp) #save $t1 sw $t0,4($sp) #save $t0 sw $s0,0($sp) #save $s0 add $t0,$a0,$a1 # $t0=g+h add $t1,$a2,$a3 # $t1=i+j sub $s0,$t0,$t1 # f=$t0+$t1 add $v0,$s0,$zero # $v0=$s0 lw $s0,0($sp) #restore $s0 lw $t0,4($sp) #restore $t0 lw $t1,8($sp) #restore $t1 addi $sp,$sp,12#remove the room on the stack jr $ra #jump back to point of origin

Computer Structure - The Instruction Set (2) Picture of Stack  MIPS software offers 2 classes of registers: $t0-$t9 : 10 temporary registers, not saved by the procedure. $s0-$s7 : 8 saved registers, saved by the procedure. 6/17

Computer Structure - The Instruction Set (2) Nested Procedures  Procedures that don't call others are called leaf procedures, procedures that call others are called nested procedures.  Problems may arise now, for example the main program calls procedure A with the argument 3 in $a0 and the return address in $ra. Procedure A then calls procedure B with the argument 7 in $a0 and with its return address in $ra. The previous address saved in $ra is destroyed.  We must, somehow, preserve these values across calls.  Lets look at a translation of the recursive factorial function in C++.

Computer Structure - The Instruction Set (2) Factorial in C++ int fact(int n) { if(n < 1) return (1); else return (n * fact(n -1)); }  The function calls itself multiple times.  The argument n is in register $a0, which is saved on the stack along with $ra. 7/17

Computer Structure - The Instruction Set (2) Factorial in Assembly fact: subi $sp,$sp,8 # make room for 2 items sw $ra,4($sp)# push the return address sw $a0,0($sp)# push the argument n slt $t0,$a0,1 # test for n<1 beq $t0,$zero,L1 # if n>=1 goto L1 li $v0,1 # pseudoinstruction $v0=1 addi $sp,$sp,8 # pop 2 items off stack jr $ra The following is the recursive call to fact(n-1) L1: subi $a0,$a0,1 # n-- jal fact # call fact(n-1) lw $a0,0($sp) # return from fact(n-1) lw $ra,4($sp) # pop n and return address addi $sp,$sp,8 # pop 2 items off stack mult $v0,$a0,$v0 # return n * fact(n-1) jr $ra

Computer Structure - The Instruction Set (2) Procedure Frame  The stack above $sp is preserved ( נשמר ) by making sure that the callee ( פונקציה נקראת ) doesn't write above $sp.  $sp is preserved by adding exactly the amount subtracted from it.  All other registers are preserved by being saved on the stack.  The stack also contains local variables that don't fit into registers.  The segment of the stack containing the saved registers and local variables is called the procedure frame or activation record.

Computer Structure - The Instruction Set (2) Frame Pointer  Some MIPS software use the register $fp to point to the first word of the procedure frame or activation record. 9/17

Computer Structure - The Instruction Set (2) Static and Automatic Variables  C++ has two storage classes for variables automatic and static.  Automatic variables are local to a function and are deleted when the function exits.  Static variables exist across function calls. Global C++ variables are static, as well as any variables defined with the keyword static. All other variables are automatic.  MIPS software reserves a register called the global pointer or $gp. Static variables are accessed through this register. 10/17

Computer Structure - The Instruction Set (2) Immediate Operands  As mentioned before the I-format instruction contains a 16 bit constant called an immediate. Using I-type instructions avoids loading values from memory into a register. Examples of instructions which use immediate values are: multi $s0,$s1,4 # $s0=$s1*4 slti $t0,$s2,10 # $t0=1 if $s2<10  But if a constant is larger than 16 bits? MIPS provides an instruction lui that loads a 16 bit constant into the upper half of an register. In order to load the value: into $s0 we must perform: lui $s0,61 #61d = addi $s0,$s0,2304 # 2304d =

Computer Structure - The Instruction Set (2) Addressing in Branches and Jumps  j # go to location Jumps to the address in memory bits (opcode) 26 bits (address)  bne $s0,$s1,Exit # branch if $s0!=$s Exit 6 bits 5 bits 5 bits 16 bits (address)  Problem: Addresses have to fit into a 16-bit field, no program could be larger than 64KByte.  Solution: Specify a register which would be added to the branch address. But which register? 11/17

Computer Structure - The Instruction Set (2) PC-Relative Addressing  The Program Counter (PC) is a register that always contains the address of the current instruction being executed.  By using the PC as the register to add to the branch address we can always branch within a range of to bytes of the current instruction.  This is enough for most loops and if statements.  This is called PC-relative addressing.  Procedures are not usually within a short range of the current instruction and thus jal is a J-type instruction. 12/17

Computer Structure - The Instruction Set (2) Branch Offset in Machine Language  Lets look at a loop in assembly: Loop:.. bne $t0,$t1,Exit subi $t0,$t0,1 j Loop Exit:  The machine code of the bne instruction is:  The branch instruction adds 8 bytes to the PC and not 12 because the PC is automatically incremented by 4 when an instruction is executed.  In fact the branch offset is 2 not 8. All MIPS instructions are 4 bytes long thus the offset is in words not bytes. The range of a branch has been multiplied by 4. 13/17

Computer Structure - The Instruction Set (2) Branch Offset Example 48subi$t0,$t1, bne$zero,$t0,L L1: jr$ra  What is the machine code translation of bne ?  opcode: 5, the opcode of bne.  rs: 0, $zero is register #0.  rt: 9, $t0 is register #9.  address: (L1 - PC)/4 = ( )/4 = 20/4 = 5  And if L1 is at address 28?  address: (L1 - PC)/4 = ( )/4 = -28/4 = -7 14/17

Computer Structure - The Instruction Set (2) Pseudodirect Addressing  The 26-bit field in the jump instruction is also a word address. Thus it is a 28-bit address. But the PC holds 32-bits?  The MIPS jump instruction replaces only the lower 28 bits of the PC, leaving the 4 highest bits of the PC unchanged.  48jL L2: add …  What is the machine code translation of j ?  opcode: 2, the opcode of j.  address: PC || /17

Computer Structure - The Instruction Set (2) Addressing Mode Summary  Immediate addressing - the Operand is a constant  Register addressing - the Operand is a register  Base or displacement addressing - the operand is at the memory location whose address is the sum of a register and a constant in the instruction.  PC-relative addressing - the address is the sum of the PC and a constant in the instruction.  Pseudodirect addressing - the jump address is the 26 bits of the instruction concatenated ( מצורף ) to the upper bits of the PC. 16/17

 Immediate addressing - the Operand is a constant addi $t0,$t1,102 # $t0=$t andi $s0,$s0,16 # $s0=$s0 & 16  Register addressing - the Operand is a register addi $t0,$t1,102 andi $s0,$s0,16 jr $s4 # jump to the address in $s4  Base or displacement addressing - the operand is at the memory location whose address is the sum of a register and a constant in the instruction. lw $t0,20($gp) # $t0 = $gp[5] lb $t1,7($s5) # $t0 = $s5[7]  PC-relative addressing - the address is the sum of the PC and a constant in the instruction. beg $s0,$s1,Label # if $s0<$s1 jump to Label  Pseudodirect addressing - the jump address is the 26 bits of the instruction concatenated ( מצורף ) to the upper bits of the PC. j L37 jal strcmp

Computer Structure - The Instruction Set (2) Addressing Modes (picture) 17/17