CS61C Fall 2004

Project 02
sprintf for MIPS

Purpose

The purpose of this project is to familiarize you with the details of the MIPS calling convention and pump up your assembly programming skills.

Administrative Requirements

To submit your project, create a directory named proj2 that contains your sprintf.s file. From within that directory, type "submit proj2".

The project deadline has been extended to 11:59pm on Friday, October 15th.

This is an individual project, not to be done in partnership. Hand in your own work, and do not collaborate with anyone else.

Disclaimer

This document mentions certain test cases. However, this input/output behavior is not a comprehensive specification of all possible behavior. Therefore, even if your project submission passes all of the test cases mentioned here, it may not be entirely correct. We strongly urge you to create test cases in addition to the ones provided here.

Frequently Asked Questions

To help you with questions you can have, please first consult the project FAQ.

Background

MIPS Stack Managing

By now, you should know how to manage the stack when writing procedures in MIPS. Let's assume you have a function light_blue, which calls a function dark_blue. The very first thing that light_blue does is to allocate space in the stack for its variables (prolog). In Figure 1 (middle) we can see that it allocates space for 4 words. light_blue eventually calls dark_blue, which allocates space for its own variables. Figure 1 (right) shows that dark_blue has allocated space for 2 words.

Figure 1

MIPS Argument Passing

The MIPS procedure calling convention uses registers $a0-$a3 for passing arguments down to procedures. If there are more than four, the remaining arguments are passed on the stack. How is this mixed with the normal stack managing? Each argument gets one word of stack space. Suppose we are trying to write in MIPS assembler a program like this:

int dark_blue (int x, int y, int quux, int bar, int baz) {

     int a;
     ...
     a = y;
     ...
}

int light_blue () {
     int c, d, e;
     ...
     dark_blue (3, 4, 43, 62, 1);
     ...
}

Procedure dark_blue has five integer arguments. The space for those arguments is allocated on the stack as part of the caller's (light_blue) stack frame. In other words, light_blue, not dark_blue, must allocate the space.

Figure 2 shows this stack management method. light_blue now allocates space in the stack for its variables and the arguments that it will use to call dark_blue. We can see in Figure 2 (middle) that it allocates space for 9 words: 4 for its own consumption, and 5 for calling dark_blue.

light_blue eventually calls dark_blue, which allocates space for its own variables. Figure 2 (right) shows that dark_blue has allocated space for 2 words. Note that dark_blue can access to the 5 arguments by accessing to the beginning of the stack reserved by its caller process (light_blue).

Figure 2

The 5 arguments (patterned light-blue stack) are located at the lower addresses of light_blue's stack frame.

That is, light_blue will use 0($sp) to store the argument x, 4($sp) to store the argument y, and so on. Therefore, dark_blue will use 8($sp) to access to the argument x, 12($sp) to access to the argument y, and so on.

Note that the first argument is always at the caller's stack frame lowest address, and the last argument at the highest address. You have to be consistent about this so that dark_blue knows which argument is which.

light_blue:
	# prolog
        addi $sp, $sp, -16  # allocate stack space for 4 words: 
                            # $ra, c, d, and e
        sw   $ra, 12($sp)   # save $ra
                            # use 0($sp) to save c ($t0) when needed
                            # use 4($sp) to save d ($t1) when needed
                            # use 8($sp) to save e ($t2) when needed

	# body

        ...

        addi $t0, $0, 3     # assume this holds the value of c
        addi $t1, $0, 4     # assume this holds the value of d
        addi $t2, $0, 5     # assume this holds the value of e

        ...

	# this is the call to dark_blue
        addi $sp, $sp, -20  # allocate stack space for 5 words (patterned 
	                    # light-blue): the args x, y, quux, bar, baz
        sw   $t0, 20($sp)   # save c before calling dark_blue
        sw   $t1, 24($sp)   # save d before calling dark_blue
        sw   $t2, 28($sp)   # save e before calling dark_blue

        add  $a0, $0, $t0   # arg x = c
        sw   $a0, 0($sp)    # (proper MIPS convention doesn't require $a0 in stack, but we do)
        add  $a1, $0, $t1   # arg y = d
        sw   $a1, 4($sp)    # (proper MIPS convention doesn't require $a1 in stack, but we do)
        addi $a2, $0, 43    # arg quux = 43
        sw   $a2, 8($sp)    # (proper MIPS convention doesn't require $a2 in stack, but we do)
        addi $a3, $0, 62    # bar = 62
        sw   $a3, 12($sp)   # (proper MIPS convention doesn't require $a3 in stack, but we do)
        addi $t0, $0, 1     # baz = 1, but no more registers
        sw   $t0, 16($sp)   # so pass on the stack
        jal  dark_blue
        addi $sp, $sp, 20   # restore stack space used for calling dark_blue

        ...

	# epilog
        lw   $ra, 12($sp)   # reload return address
        addi $sp, $sp, 16   # restore stack space
        jr   $ra            # return to caller


	...


dark_blue:
	# prolog
        addi $sp, $sp, -8   # allocate stack space for 2 words:
                            # $ra, a
        sw   $ra, 4($sp)    # save $ra
                            # use 0($sp) to save a when needed

	# body

        ...

        add  $t0, $0, $a1   # get argument y (you can also get it from the stack)
        lw   $t1, 24($sp)   # *** (see below)
                            # 8 (dark_blue's frame) + 16 = 24 up on stack
                            # fetched argument baz
        ...

	# epilog
        lw   $ra, 4($sp)    # reload return address
        addi $sp, $sp, 8    # restore stack space
        jr   $ra            # return to caller

The instruction indicated by "***" is the key to understanding the stack method of argument passing. Procedure dark_blue is referring to a word of stack memory that is from the caller's stack frame. Its own frame includes only the two words 0($sp) and 4($sp).

Project Description

Write a MIPS assembly language implementation of the C function sprintf:

int sprintf (char *outbuf, char *format, ...)

sprintf works like printf, except that it writes to the string outbuf instead of to standard output. outbuf is assumed already to point to allocated memory sufficient to hold the generated characters. Your function must accept any number of arguments, passed according to a non-MIPS standard conventions: All the arguments will be passed through the stack, with the first argument in the lowest stack position (lowest in value, i.e., closer to zero. This is equivalent to closer to the top in both Figure 1 and 2.) See the last extra note of this document to clear up how argument passing should work.

The first argument is the address of a character array into which your procedure will put its results. The second argument is a format string in which each occurrence of a percent sign (%) indicates where one of the subsequent arguments is to be substituted and how it is to be formatted. The remaining arguments are values that are to be converted to printable character form according to the format instructions. sprintf returns the number of characters in its output string not including the null at the end.

You do not have to do any error checking (e.g. comparing the number of arguments to the number of % specifications). You also do not have to implement all of the formatting options of the real sprintf. Here are the ones you are to implement:

• %d: signed integer argument to signed decimal (treat the next argument as a signed integer and output in decimal)
• %u: unsigned integer argument to unsigned decimal (treat the next argument as a unsigned integer and output in decimal)
• %b: unsigned integer argument to unsigned binary (treat the next argument as a unsigned integer and output in base 2)
• %c: character argument copied to output (treat the low byte of the argument word as a character and copy it to the output)
• %s: include a string of characters in output (treat the argument as a pointer to a null-terminated string to be copied to the output)
• %f: floating-point number (treat the argument as a 32-bit word containing a IEEE 754, single-precision floating point number, and copy its value to the output following the guidelines mentioned below)

Don't implement width or precision modifiers (e.g., %6d ). Copy the two files sprintf.s and test.sprintf.s. Or you can grab the files from your accounts in the folder ~cs61c/lib/proj2/ . Only your sprintf.s will be graded. You should modify test.sprintf.s to test your code more thoroughly.

To run this project, you need to load two files in the correct order: run xspim and load test.sprintf.s. Next load sprintf.s. Finally, run your program. The following Makefile may make your life easier.

Floating-Point Formatting

Formatting floating-point numbers is a hard problem. Therefore, we will use a different format than what the real sprintf produces.

Remember that the IEEE 754 standard defines 5 different encodings for single-precision FP numbers: zero, normalized numbers, denormalized numbers, NaN, and infinity. We want your sprintf implementation to support all of them. The good thing is that, for 4 out of the 5 encodings, you'll just copy to the output buffer the sign and a string stating what encoding uses the number (0, denorm, Inf, or NaN). You must only develop the real value for normalized numbers, and in that case, you will format the mantissa in binary, and the exponent in decimal.

Single Precision		Object Represented	What you must write into the buffer
Exponent	Mantissa
0	0	zero	[-]0
0	nonzero	± denormalized number	[-]denorm
1-254	anything	± normalized number	[-]mantissa_in_binaryb 2^([-]exponent_in_decimal)
255	0	± infinity	[-]Inf
255	nonzero	NaN (Not a Number)	[-]NaN

Consider the following piece of code:

	.data
__buffer:
	.space  200
__format:
	.asciiz "%f"
__float:
	# here goes the code in the table's left column
	***


	.text

	addi $sp, $sp, -20          # space for the stack of parameters
	la   $t0, __buffer
	sw   $t0, 0($sp)            #  0($sp): __buffer
	la   $t0, __format
	sw   $t0, 4($sp)            #  4($sp): __format
	la   $t0, __float
	lw   $t0, 0($t0)
	sw   $t0, 8($sp)            #  8($sp): __float   (%f)
	jal  sprintf

The following table shows some examples of how your sprintf code must deal with different floating point values. The left column of the table shows what goes in the line indicated by "***", and the right column what sprintf must write into the output buffer.

Code in "***"	Output Buffer
.word 0xffff0000	-NaN
.word 0x7f800000	Inf
.word 0x00100000	denorm
.word 0x80000000	-0
.float 1.2e25	1.00111101101000110010101b 2^(83)
.float 1048576.5	1.000000000000000000001b 2^(20)
.float 1048576.125	1.00000000000000000000001b 2^(20)
.float 1048576.0625	1.0b 2^(20)
.float 0.5078125	1.000001b 2^(-1)
.float -0.03125	-1.0b 2^(-5)
.float +1.125	1.001b 2^(0)
.float -1.25	-1.01b 2^(0)
.float -1.5	-1.1b 2^(0)
.float -1.0	-1.0b 2^(0)

Miscellaneous Requirements

• Obey all register conventions in this project: return the number of characters in $v0 and do not use $sx registers without saving them first. In particular, remember that the $ax registers are temporary registers, so they will be clobbered by any function call. Points will be deducted for violations of these conventions. Your sprintf procedure must work with the main function supplied in test.sprintf.s.
• You must comment your MIPS code. The graders will read it, and they need to be able to quickly understand what every section does. A common style for commenting assembly is to include a long introductory comment before every block of assembly statements, with a short comment after every line telling what it does. The introductory comment must describe the algorithm that the following block implements. The line-by-line comments must just be an easy-to-read version of the assembly code, using real variable names and perhaps more C-like constructs. If your code ends up being at least as much comment as code, this is probably a sign that you are doing a good job of commenting.
• Put your login name, section, and t.a. information at the top of sprintf.s .

Extra notes to help you:

• Minimum-Effort Digit Formatting: For all the number formatting cases, you must minimize the number of digits your sprintf prints. For example, if the user tries to format the number 19 (decimal), your sprintf must format it as "10011" in binary, and not as, say, "00010011". Note, though, that when formatting the mantissa in FP numbers, the part to the right of the decimal point must have at least one digit, i.e., sprintf must format 1.0 as "1.0b 2^(0)", not as "1.b 2^(0)".
• Unsupported Format Specifiers: Your code does not need to handle any cases that are not mentioned here. For example, if you encounter a format string with the sequence %% or %t, you must copy to the output just the last character, "%" or "t" respectively. That is, an unsupported escape sequence must just copy the character following the %. However, your code should not crash under these circumstances. It must continue processing the rest of the string.
• Spaces between '%' and format char: Apparently, the original sprintf will interpret the format string "Hello % d\n" the same as "Hello %d\n". However, in this project you will assume that if the % sign is followed by a space, this is an unsupported escape sequence, and therefore you must print the space.
• Calling Convention: Imagine that you have a subfunction that only your sprintf function calls. Could you violate the calling convention? For example, can you have the subfunction modify $t0 and leave a useful value in it for sprintf? Can you use $s6 knowing that it will not be used by other parts of sprintf? Certainly your code could work and still be violating the calling convention. However, suppose that I want to use your subfunction in my code. If I try this, it won't work. Similarly, suppose I want to insert a different version of the subfunction into your code. This will not work either because your sprintf is execting the subfunction to modify $t0 in a useful way. This may seem unlikely but as the code that you work with gets bigger and bigger, adhering to calling conventions becomes more and more important. So, when you write subfunctions, do as this spec says and OBSERVE THE CALLING CONVENTION. You will be graded on this!!
• sprintf 4 Arguments Management: Note that the we deviate slightly from the proper MIPS stack conventions (see page 88 of the P&H 3rd book). These state that the callee should always expect its first 4 arguments in the $a0-$a3 registers, and if it needs more arguments, it should get them from the stack (allocating stack space for the first 4). In other words, a proper MIPS function should not get the first 4 arguments from the caller's stack (though it has to consider the space for those 4 arguments when accessing the others, of course).

  Here is where we deviate. For this project, your sprintf function will assume that:

  • The contents of the first 4 words of the caller stack are the valid source for the first 4 parameters, instead of the $a0-$a3 registers (again, this is a clear break of the MIPS convention). You must access to the first 4 arguments through the stack.
  • The caller will always pass the parameters through the stack, even if there are less than 4 arguments (this is another clear break of the MIPS convention. Note, though, that this may help in your implementation).

  In conclusion, your sprintf must get all arguments, all the time, from the stack.