inst.eecs.berkeley.edu/~cs61c UCB CS61C : Machine Structures Lecture 9 – Introduction to MIPS Data Transfer & Decisions I 2010-02-08 Since 2001, the TIOBE programming community index has been charting the populatiry of programming languages (they use search engines). Note it isn’t calculating the best or most lines of code. Lecturer SOE Dan Garcia www.tiobe.com/index.php/content/paperinfo/tpci/ Hi to Jon Cappella from Denver, CO CS61C L09 Introduction to MIPS : Data Transfer and Decisions (2) Garcia, Spring 2010 © UCB Review In MIPS Assembly Language: Registers replace variables One Instruction (simple operation) per line Simpler is Better, Smaller is Faster New Instructions: add, addi, sub New Registers: C Variables: $s0 - $s7 Temporary Variables: $t0 - $t7 Zero: $zero CS61C L09 Introduction to MIPS : Data Transfer and Decisions (3) Garcia, Spring 2010 © UCB Assembly Operands: Memory C variables map onto registers; what about large data structures like arrays? 1 of 5 components of a computer: memory contains such data structures But MIPS arithmetic instructions only operate on registers, never directly on memory. Data transfer instructions transfer data between registers and memory: Memory to register Register to memory CS61C L09 Introduction to MIPS : Data Transfer and Decisions (4) Garcia, Spring 2010 © UCB Anatomy: 5 components of any Computer Processor Computer Control (“brain”) Datapath Registers Memory Devices Input Output These are “data transfer” instructions… Registers are in the datapath of the processor; if operands are in memory, we must transfer them to the processor to operate on them, and then transfer back to memory when done. CS61C L09 Introduction to MIPS : Data Transfer and Decisions (5) Garcia, Spring 2010 © UCB Data Transfer: Memory to Reg (1/4) To transfer a word of data, we need to specify two things: Register: specify this by # ($0 - $31) or symbolic name ($s0,…,$t0,…) Memory address: more difficult Think of memory as a single one-dimensional array, so we can address it simply by supplying a pointer to a memory address. Other times, we want to be able to offset from this pointer. Remember: “Load FROM memory” CS61C L09 Introduction to MIPS : Data Transfer and Decisions (6) Garcia, Spring 2010 © UCB Data Transfer: Memory to Reg (2/4) To specify a memory address to copy from, specify two things: A register containing a pointer to memory A numerical offset (in bytes) The desired memory address is the sum of these two values. Example: 8($t0) specifies the memory address pointed to by the value in $t0, plus 8 bytesCS61C L09 Introduction to MIPS : Data Transfer and Decisions (7) Garcia, Spring 2010 © UCB Data Transfer: Memory to Reg (3/4) Load Instruction Syntax: 1 2,3(4) where 1) operation name 2) register that will receive value 3) numerical offset in bytes 4) register containing pointer to memory MIPS Instruction Name: lw (meaning Load Word, so 32 bits or one word are loaded at a time) CS61C L09 Introduction to MIPS : Data Transfer and Decisions (8) Garcia, Spring 2010 © UCB Data Transfer: Memory to Reg (4/4) Example: lw $t0,12($s0) This instruction will take the pointer in $s0, add 12 bytes to it, and then load the value from the memory pointed to by this calculated sum into register $t0 Notes: $s0 is called the base register 12 is called the offset offset is generally used in accessing elements of array or structure: base reg points to beginning of array or structure (note offset must be a constant known at assembly time) Data flow CS61C L09 Introduction to MIPS : Data Transfer and Decisions (9) Garcia, Spring 2010 © UCB Data Transfer: Reg to Memory Also want to store from register into memory Store instruction syntax is identical to Load’s MIPS Instruction Name: sw (meaning Store Word, so 32 bits or one word is stored at a time) Example: sw $t0,12($s0) This instruction will take the pointer in $s0, add 12 bytes to it, and then store the value from register $t0 into that memory address Remember: “Store INTO memory” Data flow CS61C L09 Introduction to MIPS : Data Transfer and Decisions (10) Garcia, Spring 2010 © UCB Pointers v. Values Key Concept: A register can hold any 32-bit value. That value can be a (signed) int, an unsigned int, a pointer (memory addr), and so on E.g., If you write: add $t2,$t1,$t0 then $t0 and $t1 better contain values that can be added E.g., If you write: lw $t2,0($t0) then $t0 better contain a pointer Don’t mix these up! CS61C L09 Introduction to MIPS : Data Transfer and Decisions (11) Garcia, Spring 2010 © UCB Addressing: Byte vs. Word Every word in memory has an address, similar to an index in an array Early computers numbered words like C numbers elements of an array: Memory[0], Memory[1], Memory[2], … • Computers needed to access 8-bit bytes as well as words (4 bytes/word) • Today machines address memory as bytes, (i.e., “Byte Addressed”) hence 32-bit (4 byte) word addresses differ by 4 • Memory[0], Memory[4], Memory[8] Called the “address” of a word CS61C L09 Introduction to MIPS : Data Transfer and Decisions (12) Garcia, Spring 2010 © UCB Compilation with Memory What offset in lw to select A[5] in C? 4x5=20 to select A[5]: byte v. word Compile by hand using registers: g = h + A[5];! g: $s1, h: $s2, $s3: base address of A 1st transfer from memory to register: !lw $t0,20($s3) # $t0 gets A[5] Add 20 to $s3 to select A[5], put into $t0! Next add it to h and place in g"add $s1,$s2,$t0 # $s1 = h+A[5]CS61C L09 Introduction to MIPS : Data Transfer and Decisions (13) Garcia, Spring 2010 © UCB Notes about Memory Pitfall: Forgetting that sequential word addresses in machines with byte addressing do not differ by 1. Many an assembly language programmer has toiled over errors made by assuming that the address of the next word can be found by incrementing the address in a register by 1 instead of by the word size in bytes. Also, remember that for both lw and sw, the sum of the base address and the offset must be a multiple of 4 (to be word aligned) CS61C L09
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