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More on MIPS programs

More on MIPS programs. SPIM does not support everything supported by a general MIPS assembler. For example, .end <function_name> doesn’t work Use j $ra

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More on MIPS programs

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  1. More on MIPS programs • SPIM does not support everything supported by a general MIPS assembler. For example, • .end <function_name> doesn’t workUse j $ra • .macro doesn’t workDefine your own small functions if there is some code that you need to call pretty often. • Using multiple files • When? For large programs • Why? Organizes things easily Allows more than one person to work on the program • How? Simply load all the files into SPIM !!! • Remember There has to be exactly one function called main Labels have to be unique among all files Don’t use “hardcoded” addresses; use only labels

  2. More on MIPS Programs (cont’d) • Printing multiple lines with a single syscall • Use .ascii instead of .asciiz for the initial strings • Eg. school: .ascii “University of” .ascii “Washington \n” .asciiz “Seattle \n” .text move $v0, 4 la $a0, school syscall • Output: University of Washington Seattle • Arrays in assembly language • There are no arrays in assembly language (of course!) • All you get is a pointer to the first element of the array(ie. address of the first element) • It’s up to the assembly program writer to treat the contents of the memory following the base address as an array • Need explicit offsets or address computation • “Pointer to a string”, “pointer to an array” • Simply the address of the first element

  3. Stack Architectures • Motivation : POSTFIX notation • What is Postfix? • A way of writing mathematical expressions • As powerful as the common notation (Infix) • Easier to implement • Infix : operation is specified in between the two operandsEg. (a x b) + (c + e / f) • POSTfix : operation is specified after all operandsEg. a b x e f / c + + • How are Postfix expressions evaluated? • Use the notion of a stack • Scan postfix expression from left to right • If you see an operand push it on the stack • If you see an operation pop operands from the stack apply operation to the oerands push result back onto stack

  4. JVM - Java Virtual Machine • An abstract computing machine • Has its own set of instructions, called bytecodes • No special hardware that provides these instructions • Can simulate this machine on any computer + : Platform Independence - : Slow • Instructions have only immediate operands • no registers! • What happens to variable operands ? • Programmer explicitly pushes operands onto the stack • Instructions use operands from the top of the stack • Example: Adding in JVM • c = a + b translates to something like • ILOAD &a ILOAD &b IADD ISTORE &c

  5. JVM (cont’d) • C code: x = 5; y = x - 1; if (x < y) goto label; • JVM code: ; x = 5; SIPUSH 5 ; get 5 ISTORE &x ; store at address of x ; y = x - 1 ILOAD &x ; load x onto stack for y=x-1 SIPUSH 1 ; 1 for y=x-1 ISUB ; subtract 1 from x ISTORE &y ; save the result in y ; if (x < y) goto label; ILOAD &x ; reload x ILOAD &y ; reload y ISUB ; subtract y from x IFLT label ; jump to label if top of stack contains something less than zero

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