Homework #2 Think in C, Write in Assembly

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1 Homework #2 Think in C, Write in Assembly Assigned: Friday 06 Sep 2013 Due: Monday 16 Sep 2013 (by 23:59:59) Converting C to MIPS Assembly For each block of C code below, convert it into an equivalent block of MIPS assembly code. Each C variable becomes one MIPS register. Use registers $t0 $t9 for now. Clearly comment each line(s) of your MIPS code with the line number of the corresponding C code, and each MIPS register with its corresponding C variable. Or: Comment in C. Copy-paste the C code itself as your line comments! Note: The MIPS convention for register usage is: $a0 $a3 are input parameters (to a MIPS procedure block). Don t use any other registers to pass arguments into a function. $v0 $v3 are return values. Don t use any other registers to return values. $s0 $s9 persist across procedure calls. Each child procedure block must save/restore these registers in its own stack frame. Hence these registers are callee-saved. To avoid acquiring a bad habit early on, simply don t use the $s* registers for this HW. $t0 $t9 are temporary. Any procedure block can freely clobber any $t* register at any time, and is not obligated to restore its previous value. Now it s the parent procedure block s responsibility to save/restore them at each point-ofcall (if it cares about making them persist). So these registers are caller-saved. You don t need any stack frames for this HW, so just use the $t0 $t9 registers. Handing In By to bei.peng@ .wsu.edu. Add [CS 260] to the Subject: line. Your filename should include your own (last) name, and HW2 somewhere. For HW2 (only), your MIPS code does not need to run in MARS. o Nonetheless, running it in MARS is still the best way to test it! 1

2 1) Celsius to Fahrenheit Assume that $a0 stores the current temperature in ºC. Convert this to ºF, and leave the result in $v0. Use integer division, and don t worry about the remainder. 1 $v0 = $a0 / 5 // integer division 2 * ; 2) Simple iteration 4 int res = 1; // C global variable 5 for (int j = 0; // quiz: is j global or local? 6 j < 5; 7 ++j) 8 res = res * 2 + 1; Notes: res is a C-style global variable. That means it occupies a chunk of memory. 1 Memory is distinct from registers. o Registers are physically located inside a CPU. registers very fast. This is what makes o Memory is physically located outside the CPU, e.g. on SIMMs or disk. o To load data from, or store data to, memory requires going over the memory bus. This is what makes memory accesses (very) slow. The intent of this C loop is to make a persistent change to res s value. That generally means you must (a) load its value into a register, (b) compute a new value from it, and then (c) store the new value back to the memory chunk. Remember to do all 3 steps! 1 We ll assume a 32-bit architecture (CPU and operating system) throughout this class. Hence a C/C++/Java int is 4 bytes = 32 bits. (In 64-bit architectures, they re 64 bits, etc.) 2

3 Registers are highly transient (not persistent): they get re-used rapidly. Any value left in a register will be overwritten and lost within a few clock cycles. Hence, don t try to dedicate a register to hold res for a long time. To allocate a persistent memory location like a C global variable, use the.word directive in your.data segment. Give your.word a label to assign a name to it. N.B. This is exactly how a C compiler handles a global variable declaration. 3) Array iteration with pointer loop variable (C/C++) [Java users beware!] In this class, we emphasize pointers a lot. This is a critical learning goal of CS 260 (and any introductory computer architecture class). You cannot avoid pointers at the CPU level, because that s what the CPU really provides in chip hardware. 2 Hence, all higher-level languages that run on modern CPUs must use pointers to talk to memory. Some languages, e.g. Pascal, C/C++, C#, deliberately expose pointers, and let their programmers deal with them directly. C was successful because it allowed high-level programmers to get close to the chip level. Some languages, e.g. Common Lisp, Visual Basic, Java, Erlang, hide the pointers under the hood. You re actually using them implicitly everywhere, but the language automates the details for you. 3 C/C++ users will (or should) be familiar with the following pointer notation. But if your only experience is with Java, you may find it alien. Make a concentrated effort to catch up to your C peers. Future HWs (and CS 360/460, etc.) will just assume mastery of pointers and C pointer notation, so get used to it ASAP. 2 Every modern CPU talks over the memory bus using some kind of indirect memory addressing, which consists of a base + an offset. The base is exactly a C pointer. And that s why C has pointers. 3 Corollary: Most of these implicit pointer languages have run-time interpreters or virtual machines, and also provide automatic garbage collection. 3

4 9 char A[] = "hello, class"; // A[] is global 10 int len = 0; // len is global 11 { 12 char * P = A; // P is local 13 while (*P 14 && *P!= s ) 15 ++P; 16 len = P - A; 17 } Notes: Line 9 declares a mutable (writable) array of char with a C-style double-quoted string initializer. In MIPS, you use the.asciiz directive in your.data segment to do the same thing. This allocate a C-style string, i.e. a null-terminated array of bytes. Give it a label to name it. 4 Lines 12 and 16 rely on the C trick that the name of an array evaluates to its address (specifically, the address of its 0 th element). o MIPS uses exactly the same convention. In MIPS, the label of a.data variable evaluates to its address (at compile-time, relative to the start of the.data segment). The MIPS compiler will automatically compute the numerical value for each label. o Use la label to load the address (not the value!!) of label into a register. Line 16 further uses C pointer arithmetic. Here, the difference between two pointers or addresses of the same type T computes the distance in elements of type T. That is, C computes the difference in raw bytes, then automatically divides by sizeof T. (Here T = char, so sizeof T = 1 byte, so there s no change in the numerical result.) This is used as a clever way to compute len without using a separate counter. 4 MIPS.asciiz permits writing to the array later, so the C equivalent is a non-const array. 4

5 Lines perform two tests on the value of `*P`. o Line 13 s test `*P` is implicitly a test for non-zero (in this case, a nonnull byte). If `*P` is null ( \0 ), it evaluates to false, and the `&&` fails. o Line 14 s test compares `*P` to the ASCII value `s`. In Lines 13 14, dereferencing a pointer using the C `*` operator requires a MIPS lb `load byte` or similar load instruction. A load instruction goes over the memory bus, copies a chunk of the specified size (one byte for lb), and pulls it into a register. At the system architecture level, this is a major operation! 5 You may optimize this by doing only one load, and then using the loaded value twice. This loop shall make a persistent change to len. Remember to store its new value back to its memory chunk. 5 High-level languages like C/C++ are nicer than assembly precisely because they make these complex operations very easy to write. Don t be fooled by the compactness of C notation. A 1-character operator in C can become 1-3 instructions in MIPS! Once you fully understand MIPS, you ll appreciate C/C++/Java. 5