)
are pseudoinstructions.
SPIM S20 is a simulator that runs programs for the MIPS R2000/R3000 RISC computers.[+] SPIM can read and immediately execute files containing assembly language or MIPS executable files. SPIM is a self-contained system for running these programs and contains a debugger and interface to a few operating system services.
The architecture of the MIPS computers is simple and regular, which makes it easy to learn and understand. The processor contains 32 general-purpose registers and a well-designed instruction set that make it a propitious target for generating code in a compiler.
However, the obvious question is: why use a simulator when many people have workstations that contain a hardware, and hence significantly faster, implementation of this computer? One reason is that these workstations are not generally available. Another reason is that these machine will not persist for many years because of the rapid progress leading to new and faster computers. Unfortunately, the trend is to make computers faster by executing several instructions concurrently, which makes their architecture more difficult to understand and program. The MIPS architecture may be the epitome of a simple, clean RISC machine.
In addition, simulators can provide a better environment for low-level programming than an actual machine because they can detect more errors and provide more features than an actual computer. For example, SPIM has a X-window interface that is better than most debuggers for the actual machines.
Finally, simulators are an useful tool for studying computers and the programs that run on them. Because they are implemented in software, not silicon, they can be easily modified to add new instructions, build new systems such as multiprocessors, or simply to collect data.
The MIPS architecture, like that of most RISC computers, is difficult to program directly because of its delayed branches, delayed loads, and restricted address modes. This difficulty is tolerable since these computers were designed to be programmed in high-level languages and so present an interface designed for compilers, not programmers. A good part of the complexity results from delayed instructions. A delayed branch takes two cycles to execute. In the second cycle, the instruction immediately following the branch executes. This instruction can perform useful work that normally would have been done before the branch or it can be a nop (no operation). Similarly, delayed loads take two cycles so the instruction immediately following a load cannot use the value loaded from memory.
MIPS wisely choose to hide this complexity by implementing a virtual machine with their assembler. This virtual computer appears to have non-delayed branches and loads and a richer instruction set than the actual hardware. The assembler reorganizes (rearranges) instructions to fill the delay slots. It also simulates the additional, pseudoinstructions by generating short sequences of actual instructions.
By default, SPIM simulates the richer, virtual machine. It can also simulate
the actual hardware. We will describe the virtual machine and only mention in
passing features that do not belong to the actual hardware. In doing so, we
are following the convention of MIPS assembly language programmers (and compilers),
who routinely take advantage of the extended machine. Instructions marked with
a dagger ()
are pseudoinstructions.
SPIM provides a simple terminal and a X-window interface. Both provide equivalent functionality, but the X interface is generally easier to use and more informative.
spim, the terminal version, and xspim, the X version, have the following command-line options:
The terminal interface (spim) provides the following commands:
Most commands can be abbreviated to their unique prefix e.g., ex, re, l, ru, s, p. More dangerous commands, such as reinitialize, require a longer prefix.
Figure: X-window
interface to SPIM.
The X version of SPIM, xspim, looks different, but should operate in the same manner as spim. The X window has five panes (see Figure 1). The top pane displays the contents of the registers. It is continually updated, except while a program is running.
The next pane contains the buttons that control the simulator:
The next two panes display the memory contents. The top one shows instructions from the user and kernel text segments.[+] The first few instructions in the text segment are startup code (__start) that loads argc and argv into registers and invokes the main routine.
The lower of these two panes displays the data and stack segments. Both panes are updated as a program executes.
The bottom pane is used to display messages from the simulator. It does not display output from an executing program. When a program reads or writes, its IO appears in a separate window, called the Console, which pops up when needed.
Although SPIM faithfully simulates the MIPS computer, it is a simulator and certain things are not identical to the actual computer. The most obvious differences are that instruction timing and the memory systems are not identical. SPIM does not simulate caches or memory latency, nor does it accurate reflect the delays for floating point operations or multiplies and divides.
Another surprise (which occurs on the real machine as well) is that a pseudoinstruction expands into several machine instructions. When single-stepping or examining memory, the instructions that you see are slightly different from the source program. The correspondence between the two sets of instructions is fairly simple since SPIM does not reorganize the instructions to fill delay slots.
Comments in assembler files begin with a sharp-sign (#). Everything from the sharp-sign to the end of the line is ignored.
Identifiers are a sequence of alphanumeric characters, underbars (_), and dots (.) that do not begin with a number. Opcodes for instructions are reserved words that are not valid identifiers. Labels are declared by putting them at the beginning of a line followed by a colon, for example:
.data
item: .word 1
.text
.globl main # Must be global
main: lw $t0, item
Strings are enclosed in double-quotes ("). Special characters in strings follow the C convention:
newline \n
tab \t
quote \"
SPIM supports a subset of the assembler directives provided by the MIPS assembler:
byte
boundary. For example, .align 2 aligns the next value on a word boundary.
.align 0 turns off automatic alignment of .half, .word,
.float, and .double directives until the next .data
or .kdata directive.
values
in successive bytes of memory.
floating
point double precision numbers in successive memory locations.
floating
point single precision numbers in successive memory locations.
16-bit
quantities in successive memory halfwords.
bytes
of space in the current segment (which must be the data segment in SPIM).
32-bit
quantities in successive memory words.
SPIM provides a small set of operating-system-like services through the system
call (syscall) instruction. To request a service, a program loads the
system call code (see Table 1) into register $v0 and the arguments
into registers $a0
$a3
(or $f12 for floating point values). System calls that return values
put their result in register $v0 (or $f0 for floating point
results). For example, to print ``the answer = 5'', use the commands:
.data
str: .asciiz "the answer = "
.text
li $v0, 4 # system call code for print_str
la $a0, str # address of string to print
syscall # print the string
li $v0, 1 # system call code for print_int
li $a0, 5 # integer to print
syscall # print it
print_int is passed an integer and prints it on the console. print_float prints a single floating point number. print_double prints a double precision number. print_string is passed a pointer to a null-terminated string, which it writes to the console.
read_int, read_float, and read_double read an entire line of input up to and including the newline. Characters following the number are ignored. read_string has the same semantics as the Unix library routine fgets. It reads up to n-1 characters into a buffer and terminates the string with a null byte. If there are fewer characters on the current line, it reads through the newline and again null-terminates the string. Warning: programs that use these syscalls to read from the terminal should not use memory-mapped IO (see Section 5).
sbrk returns a pointer to a block of memory containing n additional bytes. exit stops a program from running.