``[1/ 25]th the performance at none of the cost''
[2] [2] [2] [2]
SPIM S20 is a simulator that runs programs for the MIPS R2000/R3000 RISC computers.2 SPIM can read and immediately execute files containing assembly language. 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 32-bit 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 (f) 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.
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.3 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:
SPIM does not distinguish various parts of the data segment ( .data, .rdata, and .sdata).
| Service | System Call Code | Arguments | Result |
| print_int | 1 | $a0 = integer | |
| print_float | 2 | $f12 = float | |
| print_double | 3 | $f12 = double | |
| print_string | 4 | $a0 = string | |
| read_int | 5 | integer (in $v0) | |
| read_float | 6 | float (in $f0) | |
| read_double | 7 | double (in $f0) | |
| read_string | 8 | $a0 = buffer, $a1 = length | |
| sbrk | 9 | $a0 = amount | address (in $v0) |
| exit | 10 |
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 $a0¼$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.
A MIPS processor consists of an integer processing unit (the CPU) and a collection of coprocessors that perform ancillary tasks or operate on other types of data such as floating point numbers (see Figure 2). SPIM simulates two coprocessors. Coprocessor 0 handles traps, exceptions, and the virtual memory system. SPIM simulates most of the first two and entirely omits details of the memory system. Coprocessor 1 is the floating point unit. SPIM simulates most aspects of this unit.
| Register Name | Number | Usage |
| zero | 0 | Constant 0 |
| at | 1 | Reserved for assembler |
| v0 | 2 | Expression evaluation and |
| v1 | 3 | results of a function |
| a0 | 4 | Argument 1 |
| a1 | 5 | Argument 2 |
| a2 | 6 | Argument 3 |
| a3 | 7 | Argument 4 |
| t0 | 8 | Temporary (not preserved across call) |
| t1 | 9 | Temporary (not preserved across call) |
| t2 | 10 | Temporary (not preserved across call) |
| t3 | 11 | Temporary (not preserved across call) |
| t4 | 12 | Temporary (not preserved across call) |
| t5 | 13 | Temporary (not preserved across call) |
| t6 | 14 | Temporary (not preserved across call) |
| t7 | 15 | Temporary (not preserved across call) |
| s0 | 16 | Saved temporary (preserved across call) |
| s1 | 17 | Saved temporary (preserved across call) |
| s2 | 18 | Saved temporary (preserved across call) |
| s3 | 19 | Saved temporary (preserved across call) |
| s4 | 20 | Saved temporary (preserved across call) |
| s5 | 21 | Saved temporary (preserved across call) |
| s6 | 22 | Saved temporary (preserved across call) |
| s7 | 23 | Saved temporary (preserved across call) |
| t8 | 24 | Temporary (not preserved across call) |
| t9 | 25 | Temporary (not preserved across call) |
| k0 | 26 | Reserved for OS kernel |
| k1 | 27 | Reserved for OS kernel |
| gp | 28 | Pointer to global area |
| sp | 29 | Stack pointer |
| fp | 30 | Frame pointer |
| ra | 31 | Return address (used by function call) |
The MIPS (and SPIM) central processing unit contains 32 general purpose 32-bit registers that are numbered 0--31. Register n is designated by $n. Register $0 always contains the hardwired value 0. MIPS has established a set of conventions as to how registers should be used. These suggestions are guidelines, which are not enforced by the hardware. However a program that violates them will not work properly with other software. Table 2 lists the registers and describes their intended use.
Registers $at (1), $k0 (26), and $k1 (27) are reserved for use by the assembler and operating system.
Registers $a0--$a3 (4--7) are used to pass the first four arguments to routines (remaining arguments are passed on the stack). Registers $v0 and $v1 (2, 3) are used to return values from functions. Registers $t0--$t9 (8--15, 24, 25) are caller-saved registers used for temporary quantities that do not need to be preserved across calls. Registers $s0-- $s7 (16--23) are callee-saved registers that hold long-lived values that should be preserved across calls.
Register $sp (29) is the stack pointer, which points to the last location in use on the stack.4 Register $fp (30) is the frame pointer.5 Register $ra (31) is written with the return address for a call by the jal instruction.
Register $gp (28) is a global pointer that points into the middle of a 64K block of memory in the heap that holds constants and global variables. The objects in this heap can be quickly accessed with a single load or store instruction.
In addition, coprocessor 0 contains registers that are useful to handle exceptions. SPIM does not implement all of these registers, since they are not of much use in a simulator or are part of the memory system, which is not implemented. However, it does provide the following:
Register Name Number Usage
BadVAddr 8 Memory address at which address exception occurred
Status 12 Interrupt mask and enable bits
Cause 13 Exception type and pending interrupt bits
EPC 14 Address of instruction that caused exception
These registers are part of coprocessor 0's register set and are accessed by the lwc0, mfc0, mtc0, and swc0 instructions.
Figure 3 describes the bits in the Status register that are implemented by SPIM. The interrupt mask contains a bit for each of the five interrupt levels. If a bit is one, interrupts at that level are allowed. If the bit is zero, interrupts at that level are disabled. The low six bits of the Status register implement a three-level stack for the kernel/user and interrupt enable bits. The kernel/user bit is 0 if the program was running in the kernel when the interrupt occurred and 1 if it was in user mode. If the interrupt enable bit is 1, interrupts are allowed. If it is 0, they are disabled. At an interrupt, these six bits are shifted left by two bits, so the current bits become the previous bits and the previous bits become the old bits. The current bits are both set to 0 (i.e., kernel mode with interrupts disabled).
Figure 4 describes the bits in the Cause registers. The five pending interrupt bits correspond to the five interrupt levels. A bit becomes 1 when an interrupt at its level has occurred but has not been serviced. The exception code register contains a code from the following table describing the cause of an exception.
Number Name Description
0 INT External interrupt
4 ADDRL Address error exception (load or instruction fetch)
5 ADDRS Address error exception (store)
6 IBUS Bus error on instruction fetch
7 DBUS Bus error on data load or store
8 SYSCALL Syscall exception
9 BKPT Breakpoint exception
10 RI Reserved instruction exception
12 OVF Arithmetic overflow exception
Processors can number the bytes within a word to make the byte with the lowest number either the leftmost or rightmost one. The convention used by a machine is its byte order . MIPS processors can operate with either big-endian byte order:
| Byte # | |||
| 0 | 1 | 2 | 3 |
or little-endian byte order:
| Byte # | |||
| 3 | 2 | 1 | 0 |
SPIM operates with both byte orders. SPIM's byte order is determined by the byte order of the underlying hardware running the simulator. On a DECstation 3100, SPIM is little-endian, while on a HP Bobcat, Sun 4 or PC/RT, SPIM is big-endian.
MIPS is a load/store architecture, which means that only load and store instructions access memory. Computation instructions operate only on values in registers. The bare machine provides only one memory addressing mode: c(rx), which uses the sum of the immediate (integer) c and the contents of register rx as the address. The virtual machine provides the following addressing modes for load and store instructions:
Format Address Computation
(register) contents of register
imm immediate
imm (register) immediate + contents of register
symbol address of symbol
symbol ± imm address of symbol + or - immediate
symbol ± imm (register) address of symbol + or -
(immediate + contents of register)
Most load and store instructions operate only on aligned data. A quantity is aligned if its memory address is a multiple of its size in bytes. Therefore, a halfword object must be stored at even addresses and a full word object must be stored at addresses that are a multiple of 4. However, MIPS provides some instructions for manipulating unaligned data.
In all instructions below, Src2 can either be a register or an immediate value (a 16 bit integer). The immediate forms of the instructions are only included for reference. The assembler will translate the more general form of an instruction (e.g., add) into the immediate form (e.g., addi) if the second argument is constant.
f
abs Rdest, RsrcAbsolute Value
Put the absolute value of the integer from register Rsrc in
register Rdest.
add Rdest, Rsrc1, Src2Addition (with overflow)
addi Rdest, Rsrc1, ImmAddition Immediate (with overflow)
addu Rdest, Rsrc1, Src2Addition (without overflow)
addiu Rdest, Rsrc1, ImmAddition Immediate (without overflow)
Put the sum of the integers from register Rsrc1 and
Src2 (or Imm) into register Rdest.
and Rdest, Rsrc1, Src2AND
andi Rdest, Rsrc1, ImmAND Immediate
Put the logical AND of the integers from register Rsrc1 and
Src2 (or Imm) into register Rdest.
div Rsrc1, Rsrc2Divide (signed)
divu Rsrc1, Rsrc2Divide (unsigned)
Divide the contents of the two registers.
divu treats is operands as unsigned values. Leave the quotient in
register lo and the remainder in register hi. Note that
if an operand is negative, the remainder is unspecified by the MIPS
architecture and depends on the conventions of the machine on which
SPIM is run.
f
div Rdest, Rsrc1, Src2Divide (signed, with overflow)
f
divu Rdest, Rsrc1, Src2Divide (unsigned, without overflow)
Put the quotient of the integers from register Rsrc1 and
Src2 into register Rdest. divu treats is operands as
unsigned values.
f
mul Rdest, Rsrc1, Src2Multiply (without overflow)
f
mulo Rdest, Rsrc1, Src2Multiply (with overflow)
f
mulou Rdest, Rsrc1, Src2Unsigned Multiply (with overflow)
Put the product of the integers from register Rsrc1 and
Src2 into register Rdest.
mult Rsrc1, Rsrc2Multiply
multu Rsrc1, Rsrc2Unsigned Multiply
Multiply the contents of the two registers. Leave the low-order word
of the product in register lo and the high-word in register
hi.
f
neg Rdest, RsrcNegate Value (with overflow)
f
negu Rdest, RsrcNegate Value (without overflow)
Put the negative of the integer from register Rsrc into
register Rdest.
nor Rdest, Rsrc1, Src2NOR
Put the logical NOR of the integers from register Rsrc1 and
Src2 into register Rdest.
f
not Rdest, RsrcNOT
Put the bitwise logical negation of the integer from register
Rsrc into register Rdest.
or Rdest, Rsrc1, Src2OR
ori Rdest, Rsrc1, ImmOR Immediate
Put the logical OR of the integers from register Rsrc1 and
Src2 (or Imm) into register Rdest.
f
rem Rdest, Rsrc1, Src2Remainder
f
remu Rdest, Rsrc1, Src2Unsigned Remainder
Put the remainder from dividing the integer in register Rsrc1 by
the integer in Src2 into register Rdest. Note that if an
operand is negative, the remainder is unspecified by the MIPS
architecture and depends on the conventions of the machine on which
SPIM is run.
f
rol Rdest, Rsrc1, Src2Rotate Left
f
ror Rdest, Rsrc1, Src2Rotate Right
Rotate the contents of register Rsrc1 left (right) by the
distance indicated by Src2 and put the result in register
Rdest.
sll Rdest, Rsrc1, Src2Shift Left Logical
sllv Rdest, Rsrc1, Rsrc2Shift Left Logical Variable
sra Rdest, Rsrc1, Src2Shift Right Arithmetic
srav Rdest, Rsrc1, Rsrc2Shift Right Arithmetic Variable
srl Rdest, Rsrc1, Src2Shift Right Logical
srlv Rdest, Rsrc1, Rsrc2Shift Right Logical Variable
Shift the contents of register Rsrc1 left (right) by the
distance indicated by Src2 (Rsrc2) and put the
result in register Rdest.
sub Rdest, Rsrc1, Src2Subtract (with overflow)
subu Rdest, Rsrc1, Src2Subtract (without overflow)
Put the difference of the integers from register Rsrc1 and
Src2 into register Rdest.
xor Rdest, Rsrc1, Src2XOR
xori Rdest, Rsrc1, ImmXOR Immediate
Put the logical XOR of the integers from register Rsrc1 and
Src2 (or Imm) into register Rdest.
f
li Rdest, immLoad Immediate
Move the immediate imm into register Rdest.
lui Rdest, immLoad Upper Immediate
Load the lower halfword of the immediate imm into the upper
halfword of register Rdest. The lower bits of the register are
set to 0.
In all instructions below, Src2 can either be a register or an immediate value (a 16 bit integer).
f
seq Rdest, Rsrc1, Src2Set Equal
Set register Rdest to 1 if register Rsrc1 equals
Src2 and to be 0 otherwise.
f
sge Rdest, Rsrc1, Src2Set Greater Than Equal
f
sgeu Rdest, Rsrc1, Src2Set Greater Than Equal Unsigned
Set register Rdest to 1 if register Rsrc1 is greater
than or equal to Src2 and to 0 otherwise.
f
sgt Rdest, Rsrc1, Src2Set Greater Than
f
sgtu Rdest, Rsrc1, Src2Set Greater Than Unsigned
Set register Rdest to 1 if register Rsrc1 is greater
than Src2 and to 0 otherwise.
f
sle Rdest, Rsrc1, Src2Set Less Than Equal
f
sleu Rdest, Rsrc1, Src2Set Less Than Equal Unsigned
Set register Rdest to 1 if register Rsrc1 is less than
or equal to Src2 and to 0 otherwise.
slt Rdest, Rsrc1, Src2Set Less Than
slti Rdest, Rsrc1, ImmSet Less Than Immediate
sltu Rdest, Rsrc1, Src2Set Less Than Unsigned
sltiu Rdest, Rsrc1, ImmSet Less Than Unsigned Immediate
Set register Rdest to 1 if register Rsrc1 is less than
Src2 (or Imm) and to 0 otherwise.
f
sne Rdest, Rsrc1, Src2Set Not Equal
Set register Rdest to 1 if register Rsrc1 is not equal
to Src2 and to 0 otherwise.
In all instructions below, Src2 can either be a register or an immediate value (integer). Branch instructions use a signed 16-bit offset field; hence they can jump 215-1 instructions (not bytes) forward or 215 instructions backwards. The jump instruction contains a 26 bit address field.
f
b labelBranch instruction
Unconditionally branch to the instruction at the label.
bczt labelBranch Coprocessor z True
bczf labelBranch Coprocessor z False
Conditionally branch to the instruction at the label if coprocessor
z's condition flag is true (false).
beq Rsrc1, Src2, labelBranch on Equal
Conditionally branch to the instruction at the label if the contents
of register Rsrc1 equals Src2.
f
beqz Rsrc, labelBranch on Equal Zero
Conditionally branch to the instruction at the label if the contents
of Rsrc equals 0.
f
bge Rsrc1, Src2, labelBranch on Greater Than Equal
f
bgeu Rsrc1, Src2, labelBranch on GTE Unsigned
Conditionally branch to the instruction at the label if the contents
of register Rsrc1 are greater than or equal to Src2.
bgez Rsrc, labelBranch on Greater Than Equal Zero
Conditionally branch to the instruction at the label if the contents
of Rsrc are greater than or equal to 0.
bgezal Rsrc, labelBranch on Greater Than Equal Zero And Link
Conditionally branch to the instruction at the label if the contents
of Rsrc are greater than or equal to 0. Save the address of
the next instruction in register 31.
f
bgt Rsrc1, Src2, labelBranch on Greater Than
f
bgtu Rsrc1, Src2, labelBranch on Greater Than Unsigned
Conditionally branch to the instruction at the label if the contents
of register Rsrc1 are greater than Src2.
bgtz Rsrc, labelBranch on Greater Than Zero
Conditionally branch to the instruction at the label if the contents
of Rsrc are greater than 0.
f
ble Rsrc1, Src2, labelBranch on Less Than Equal
f
bleu Rsrc1, Src2, labelBranch on LTE Unsigned
Conditionally branch to the instruction at the label if the contents
of register Rsrc1 are less than or equal to Src2.
blez Rsrc, labelBranch on Less Than Equal Zero
Conditionally branch to the instruction at the label if the contents
of Rsrc are less than or equal to 0.
bgezal Rsrc, labelBranch on Greater Than Equal Zero And Link
bltzal Rsrc, labelBranch on Less Than And Link
Conditionally branch to the instruction at the label if the contents
of Rsrc are greater or equal to 0 or less than 0,
respectively. Save the address of the next instruction in register 31.
f
blt Rsrc1, Src2, labelBranch on Less Than
f
bltu Rsrc1, Src2, labelBranch on Less Than Unsigned
Conditionally branch to the instruction at the label if the contents
of register Rsrc1 are less than Src2.
bltz Rsrc, labelBranch on Less Than Zero
Conditionally branch to the instruction at the label if the contents
of Rsrc are less than 0.
bne Rsrc1, Src2, labelBranch on Not Equal
Conditionally branch to the instruction at the label if the contents
of register Rsrc1 are not equal to Src2.
f
bnez Rsrc, labelBranch on Not Equal Zero
Conditionally branch to the instruction at the label if the contents
of Rsrc are not equal to 0.
j labelJump
Unconditionally jump to the instruction at the label.
jal labelJump and Link
jalr RsrcJump and Link Register
Unconditionally jump to the instruction at the label or whose address
is in register Rsrc. Save the address of the next
instruction in register 31.
jr RsrcJump Register
Unconditionally jump to the instruction whose address is in register
Rsrc.
f
la Rdest, addressLoad Address
Load computed address , not the contents of the location, into
register Rdest.
lb Rdest, addressLoad Byte
lbu Rdest, addressLoad Unsigned Byte
Load the byte at address into register Rdest. The byte
is sign-extended by the lb, but not the lbu, instruction.
f
ld Rdest, addressLoad Double-Word
Load the 64-bit quantity at address into registers Rdest
and Rdest + 1.
lh Rdest, addressLoad Halfword
lhu Rdest, addressLoad Unsigned Halfword
Load the 16-bit quantity (halfword) at address into register
Rdest. The halfword is sign-extended by the lh, but not
the lhu, instruction
lw Rdest, addressLoad Word
Load the 32-bit quantity (word) at address into register
Rdest.
lwcz Rdest, addressLoad Word Coprocessor
Load the word at address into register Rdest of
coprocessor z (0--3).
lwl Rdest, addressLoad Word Left
lwr Rdest, addressLoad Word Right
Load the left (right) bytes from the word at the possibly-unaligned
address into register Rdest.
f
ulh Rdest, addressUnaligned Load Halfword
f
ulhu Rdest, addressUnaligned Load Halfword Unsigned
Load the 16-bit quantity (halfword) at the possibly-unaligned
address into register Rdest. The halfword is sign-extended
by the ulh, but not the ulhu, instruction
f
ulw Rdest, addressUnaligned Load Word
Load the 32-bit quantity (word) at the possibly-unaligned
address into register Rdest.
sb Rsrc, addressStore Byte
Store the low byte from register Rsrc at address .
f
sd Rsrc, addressStore Double-Word
Store the 64-bit quantity in registers Rsrc and Rsrc
+ 1 at address .
sh Rsrc, addressStore Halfword
Store the low halfword from register Rsrc at address .
sw Rsrc, addressStore Word
Store the word from register Rsrc at address .
swcz Rsrc, addressStore Word Coprocessor
Store the word from register Rsrc of coprocessor z at
address .
swl Rsrc, addressStore Word Left
swr Rsrc, addressStore Word Right
Store the left (right) bytes from register Rsrc at the
possibly-unaligned address .
f
ush Rsrc, addressUnaligned Store Halfword
Store the low halfword from register Rsrc at the
possibly-unaligned address .
f
usw Rsrc, addressUnaligned Store Word
Store the word from register Rsrc at the possibly-unaligned
address .
f
move Rdest, RsrcMove
Move the contents of Rsrc to Rdest.
The multiply and divide unit produces its result in two additional
registers, hi and lo. These instructions move values to and from
these registers. The multiply, divide, and remainder instructions
described above are pseudoinstructions that make it appear as if this
unit operates on the general registers and detect error conditions
such as divide by zero or overflow.
mfhi RdestMove From hi
mflo RdestMove From lo
Move the contents of the hi (lo) register to register Rdest.
mthi RdestMove To hi
mtlo RdestMove To lo
Move the contents register Rdest to the hi (lo) register.
Coprocessors have their own register sets. These instructions move
values between these registers and the CPU's registers.
mfcz Rdest, CPsrcMove From Coprocessor z
Move the contents of coprocessor z's register CPsrc to CPU
register Rdest.
f
mfc1.d Rdest, FRsrc1Move Double From Coprocessor 1
Move the contents of floating point registers FRsrc1 and
FRsrc1 + 1 to CPU registers Rdest and Rdest + 1.
mtcz Rsrc, CPdestMove To Coprocessor z
Move the contents of CPU register Rsrc to coprocessor z's
register CPdest.
The MIPS has a floating point coprocessor (numbered 1) that operates on single precision (32-bit) and double precision (64-bit) floating point numbers. This coprocessor has its own registers, which are numbered $f0--$f31. Because these registers are only 32-bits wide, two of them are required to hold doubles. To simplify matters, floating point operations only use even-numbered registers--including instructions that operate on single floats.
Values are moved in or out of these registers a word (32-bits) at a time by lwc1, swc1, mtc1, and mfc1 instructions described above or by the l.s, l.d, s.s, and s.d pseudoinstructions described below. The flag set by floating point comparison operations is read by the CPU with its bc1t and bc1f instructions.
In all instructions below, FRdest, FRsrc1, FRsrc2, and FRsrc are floating point registers (e.g., $f2).
abs.d FRdest, FRsrcFloating Point Absolute Value Double
abs.s FRdest, FRsrcFloating Point Absolute Value Single
Compute the absolute value of the floating float double (single) in
register FRsrc and put it in register FRdest.
add.d FRdest, FRsrc1, FRsrc2Floating Point Addition Double
add.s FRdest, FRsrc1, FRsrc2Floating Point Addition Single
Compute the sum of the floating float doubles (singles) in registers
FRsrc1 and FRsrc2 and put it in register FRdest.
c.eq.d FRsrc1, FRsrc2Compare Equal Double
c.eq.s FRsrc1, FRsrc2Compare Equal Single
Compare the floating point double in register FRsrc1 against
the one in FRsrc2 and set the floating point condition flag
true if they are equal.
c.le.d FRsrc1, FRsrc2Compare Less Than Equal Double
c.le.s FRsrc1, FRsrc2Compare Less Than Equal Single
Compare the floating point double in register FRsrc1 against
the one in FRsrc2 and set the floating point condition flag
true if the first is less than or equal to the second.
c.lt.d FRsrc1, FRsrc2Compare Less Than Double
c.lt.s FRsrc1, FRsrc2Compare Less Than Single
Compare the floating point double in register FRsrc1 against
the one in FRsrc2 and set the condition flag true if the first
is less than the second.
cvt.d.s FRdest, FRsrcConvert Single to Double
cvt.d.w FRdest, FRsrcConvert Integer to Double
Convert the single precision floating point number or integer in
register FRsrc to a double precision number and put it in
register FRdest.
cvt.s.d FRdest, FRsrcConvert Double to Single
cvt.s.w FRdest, FRsrcConvert Integer to Single
Convert the double precision floating point number or integer in
register FRsrc to a single precision number and put it in
register FRdest.
cvt.w.d FRdest, FRsrcConvert Double to Integer
cvt.w.s FRdest, FRsrcConvert Single to Integer
Convert the double or single precision floating point number in
register FRsrc to an integer and put it in register
FRdest.
div.d FRdest, FRsrc1, FRsrc2Floating Point Divide Double
div.s FRdest, FRsrc1, FRsrc2Floating Point Divide Single
Compute the quotient of the floating float doubles (singles) in
registers FRsrc1 and FRsrc2 and put it in register
FRdest.
f
l.d FRdest, addressLoad Floating Point Double
f
l.s FRdest, addressLoad Floating Point Single
Load the floating float double (single) at address into register
FRdest.
mov.d FRdest, FRsrcMove Floating Point Double
mov.s FRdest, FRsrcMove Floating Point Single
Move the floating float double (single) from register FRsrc to
register FRdest.
mul.d FRdest, FRsrc1, FRsrc2Floating Point Multiply Double
mul.s FRdest, FRsrc1, FRsrc2Floating Point Multiply Single
Compute the product of the floating float doubles (singles) in
registers FRsrc1 and FRsrc2 and put it in register
FRdest.
neg.d FRdest, FRsrcNegate Double
neg.s FRdest, FRsrcNegate Single
Negate the floating point double (single) in register FRsrc
and put it in register FRdest.
f
s.d FRdest, addressStore Floating Point Double
f
s.s FRdest, addressStore Floating Point Single
Store the floating float double (single) in register FRdest at
address.
sub.d FRdest, FRsrc1, FRsrc2Floating Point Subtract Double
sub.s FRdest, FRsrc1, FRsrc2Floating Point Subtract Single
Compute the difference of the floating float doubles (singles) in
registers FRsrc1 and FRsrc2 and put it in register
FRdest.
rfeReturn From Exception
Restore the Status register.
syscallSystem Call
Register $v0 contains the number of the system call (see
Table 1) provided by SPIM.
break nBreak
Cause exception n. Exception 1 is reserved for the debugger.
nopNo operation
Do nothing.
The organization of memory in MIPS systems is conventional. A program's address space is composed of three parts (see Figure 5).
At the bottom of the user address space (0x400000) is the text segment, which holds the instructions for a program.
Above the text segment is the data segment (starting at 0x10000000), which is divided into two parts. The static data portion contains objects whose size and address are known to the compiler and linker. Immediately above these objects is dynamic data. As a program allocates space dynamically (i.e., by malloc), the sbrk system call moves the top of the data segment up.
The program stack resides at the top of the address space (0x7fffffff). It grows down, towards the data segment.
The calling convention described in this section is the one used by gcc , not the native MIPS compiler, which uses a more complex convention that is slightly faster.
Figure 6 shows a diagram of a stack frame. A frame consists of the memory between the frame pointer ($fp), which points to the word immediately after the last argument passed on the stack, and the stack pointer ($sp), which points to the last word in the frame. As typical of Unix systems, the stack grows down from higher memory addresses, so the frame pointer is above stack pointer.
The following steps are necessary to effect a call:
Within the called routine, the following steps are necessary:
Finally, to return from a call, a function places the returned value into $v0 and executes the following steps:
In addition to simulating the basic operation of the CPU and operating system, SPIM also simulates a memory-mapped terminal connected to the machine. When a program is ``running,'' SPIM connects its own terminal (or a separate console window in xspim) to the processor. The program can read characters that you type while the processor is running. Similarly, if SPIM executes instructions to write characters to the terminal, the characters will appear on SPIM's terminal or console window. One exception to this rule is control-C: it is not passed to the processor, but instead causes SPIM to stop simulating and return to command mode. When the processor stops executing (for example, because you typed control-C or because the machine hit a breakpoint), the terminal is reconnected to SPIM so you can type SPIM commands. To use memory-mapped IO, spim or xspim must be started with the -mapped_io flag.
The terminal device consists of two independent units: a receiver and a transmitter . The receiver unit reads characters from the keyboard as they are typed. The transmitter unit writes characters to the terminal's display. The two units are completely independent. This means, for example, that characters typed at the keyboard are not automatically ``echoed'' on the display. Instead, the processor must get an input character from the receiver and re-transmit it to echo it.
The processor accesses the terminal using four memory-mapped device registers, as shown in Figure 7. ``Memory-mapped'' means that each register appears as a special memory location. The Receiver Control Register is at location 0xffff0000; only two of its bits are actually used. Bit 0 is called ``ready'': if it is one it means that a character has arrived from the keyboard but has not yet been read from the receiver data register. The ready bit is read-only: attempts to write it are ignored. The ready bit changes automatically from zero to one when a character is typed at the keyboard, and it changes automatically from one to zero when the character is read from the receiver data register.
Bit one of the Receiver Control Register is ``interrupt enable''. This bit may be both read and written by the processor. The interrupt enable is initially zero. If it is set to one by the processor, an interrupt is requested by the terminal on level zero whenever the ready bit is one. For the interrupt actually to be received by the processor, interrupts must be enabled in the status register of the system coprocessor (see Section 2).
Other bits of the Receiver Control Register are unused: they always read as zeroes and are ignored in writes.
The second terminal device register is the Receiver Data Register (at address 0xffff0004). The low-order eight bits of this register contain the last character typed on the keyboard, and all the other bits contain zeroes. This register is read-only and only changes value when a new character is typed on the keyboard. Reading the Receiver Data Register causes the ready bit in the Receiver Control Register to be reset to zero.
The third terminal device register is the Transmitter Control Register (at address 0xffff0008). Only the low-order two bits of this register are used, and they behave much like the corresponding bits of the Receiver Control Register. Bit 0 is called ``ready'' and is read-only. If it is one it means the transmitter is ready to accept a new character for output. If it is zero it means the transmitter is still busy outputting the previous character given to it. Bit one is ``interrupt enable''; it is readable and writable. If it is set to one, then an interrupt will be requested on level one whenever the ready bit is one.
The final device register is the Transmitter Data Register (at address 0xffff000c). When it is written, the low-order eight bits are taken as an ASCII character to output to the display. When the Transmitter Data Register is written, the ready bit in the Transmitter Control Register will be reset to zero. The bit will stay zero until enough time has elapsed to transmit the character to the terminal; then the ready bit will be set back to one again. The Transmitter Data Register should only be written when the ready bit of the Transmitter Control Register is one; if the transmitter isn't ready then writes to the Transmitter Data Register are ignored (the write appears to succeed but the character will not be output).
In real computers it takes time to send characters over the serial lines that connect terminals to computers. These time lags are simulated by SPIM. For example, after the transmitter starts transmitting a character, the transmitter's ready bit will become zero for a while. SPIM measures this time in instructions executed, not in real clock time. This means that the transmitter will not become ready again until the processor has executed a certain number of instructions. If you stop the machine and look at the ready bit using SPIM, it will not change. However, if you let the machine run then the bit will eventually change back to one.
1 I grateful to the many students at UW who used SPIM in their courses and happily found bugs in a professor's code. In particular, the students in CS536, Spring 1990, painfully found the last few bugs in an ``already-debugged'' simulator. I am grateful for their patience and persistence. Alan Yuen-wui Siow wrote the X-window interface.
2 For a description of the real machines, see Gerry Kane and Joe Heinrich, MIPS RISC Architecture, Prentice Hall, 1992.
3 These instructions are real--not pseudo--MIPS instructions. SPIM translates assembler pseudoinstructions to 1--3 MIPS instructions before storing the program in memory. Each source instruction appears as a comment on the first instruction to which it is translated.
4 In earlier version of SPIM, $sp was documented as pointing at the first free word on the stack (not the last word of the stack frame). Recent MIPS documents have made it clear that this was an error. Both conventions work equally well, but we choose to follow the real system.
5 The MIPS compiler does not use a frame pointer, so this register is used as callee-saved register $s8.