I wrote this code (this is only a part of it):
beq $t4 ,$0 ,__less3
add $s2,$t3,$0 # s2=t3
add $s3,$t2,$0 # s3=t2
j __next1
__less3:
add $s2,$t2,$0 # s2=t2
add $s3,$t3,$0 # s3=t3
__next1:
slt $t4, $t1, $t0 # t4=(t1<t0)
beq $t4 ,$0 ,__les1sk
The problem is that when the simulation gets to line 4 and need to do the jump, it does line 7 (the next command) and only then it jumps to line 10.
I know it because that register $s2 changes to $t2 instead of remaining with the value from $t3.
MIPS, the processor that PCSpim simulates, employs "delayed branching": it executes the instruction immediately following the branch before branching to the jump target (hence "delays" the jump). This is an efficiency measure; since the processor has gone through most of the work for that "extra" instruction, that effort won't be wasted. Compilers account for this when producing code.
I believe that there is an option in the simulator to disable delayed branching.
Related
I am having a hard time figuring out where to start with this project. I am needing to write code in PLP that is a palindrome checker.
the task is to write a program that recieves a string of characters via UART, checks if this string is a palindrome, then uses a print function to print either"yes" of "no". I have been given a template that I am to follow when creating the program.
The template project file contains six function stubs that need to be implemented. five are called from the main loop and the sixth is called from "period_check: in the template file it contains descriptions of what each function needs to do and how it should be implemented. I have attempted to fill in some, however I do not think I am on the right track. Please help.
***** I have gotten this much code in, but it does not print out the right output****
it prints no for everything vs no for non palindromes and yes for palindrome.
.org 0x10000000
# Initializations
# NOTE: You may add initializations after line 10, but please do not
# remove or change the initializations to $sp, $s0, $s1, or $s2
li $sp, 0x10fffffc # Starting address of empty stack
li $s0, 0xf0000000 # UART base address
li $s1, array_ptr # Array head pointer
li $s2, array_ptr # Array tail pointer
####################################################################
# Do not make changes to the jump to main, the allocation of
# memory for the array, or the main loop
####################################################################
j main
nop
array_ptr: # Label pointing to 100 word array
.space 100
main:
jal poll_UART
nop
jal period_check
nop
jal space_check
nop
jal case_check
nop
jal array_push
nop
j main
nop
####################################################################
# ******************************************************************
####################################################################
# The "poll_UART" function should poll the status register of the UART.
# If the 2^1 bit position (ready bit) is set to 1 then it
# should copy the receive buffer's value into $v0 and send
# a clear status command (2^1) to the command register before
# returning (a return statement is already included). In order to
# receive full credit, $s0 must contain the base address of the UART
# and must be used with the appropriate offsets to access UART
# registers and buffers
poll_UART:
lw $t1, 4($s0)
li $t2, 0b10
and $t3, $t1, $t2
beq $t3, $0, main
nop
lw $v0, 8($s0)
sw $t2, 0($s0)
jr $ra
nop
# The "period_check" function should check if the current character ($v0)
# is a period ("."). If it is a period then the function should go to the
# label, "palindrome_check". If the character is not a period then it
# should use the included return.
period_check:
li $t0, 0x2E
beq $v0, $t0, palindrome_check
nop
# The "space_check" function should check if the current character ($v0)
# is a space (" "). If it is then it should jump to "main" so
# that it skips saving the space character. If not it should
# use the included return.
space_check:
li $t4, 0x20
beq $t4, $v0, main
jr $ra
nop
# The "case_check" function should perform a single inequality check.
# If the current character ($v0) is greater than the ASCII value of 'Z',
# which indicates the current character is lowercase, then it should convert
# the value of $v0 to the uppercase equivalent and then return. If the
# current character ($v0) is already uppercase (meaning the inequality
# mentioned before was not true) then the function should return without
# performing a conversion.
case_check:
li $t5, 0x5A
slt $t6, $v0, $t5
li $t7, 1
beq $t6, $t7, convert
convert:
addiu $v0, $v0, -32
jr $ra
nop
# The "array_push" function should save the current character ($v0) to the
# current location of the tail pointer, $s2. Then it should increment the
# tail pointer so that it points to the next element of the array. Last
# it should use the included return statement.
array_push:
sw $v0, 0($s2)
addiu, $s2, $s2, 4
jr $ra
nop
# The "palindrome_check" subroutine should be jumped to by the period
# check function if a period is encountered. This subroutine should contain
# a loop that traverses the array from the front towards the back (using the
# head pointer, $s1) and from the back towards the front(using the tail
# pointer, $s2). If the string is a palindrome then as the array is traversed
# the characters pointed to should be equal. If the characters are not equal
# then the string is not a palindrome and the print function should be used
# to print "No". If the pointers cross (i.e. the head pointer's address is
# greater than or equal to the tail pointer's address) and the compared
# characters are equal then the string is a palindrome and "Yes" should be
# printed.
#
# Remember to restore the head and tail pointers to the first element
# of the array before the subroutine jumps back to main to begin processing the
# next string. Also, keep in mind that because the tail pointer is updated at
# the end of "array_push" it technically points one element past the last
# character in the array. You will need to compensate for this by either
# decrementing the pointer once at the start of the array or using an offset
# from this pointer's address.
palindrome_check:
addiu $s2, $s2, -8
move $s3, $s1
subu $s6, $s2, $s3
beq $s6, $0, palindrome
nop
check_loop:
lw $s4, 0($s3)
lw $s5, 0($s2)
bne $s5, $t0, not_palindrome
nop
adjust_pointers:
addiu $s2, $s2, -4
addiu $s3, $s3, 4
slt $t8, $s3, $s2
bne $t8, $t0, check_loop
nop
j palindrome
nop
palindrome:
li $a0, 1
call project3_print
move $s2, $s1
j main
not_palindrome:
li $a0, 0
call project3_print
move $s2, $s1
j main
nop
Ok, this is just my opinion, but you are definitely not on the right track.
The control flow you're showing is problematic.
To see one reason why, try writing this same in C or any other language that you know. You won't be able to do it because of the non-local goto's that's using, where one procedure jumps (without calling) to another procedure.
Further, finding whether an input is a palindrome is not a fixed sequence of one-time steps that are executed on each input character.
You will (1) need to store the characters for later comparison, and (2) need a decision point where you can determine (and print) yes it is, or no it isn't. You don't have any control structure for that.
that recieves a string of characters via UART, checks if this string is a palindrome, then uses a print function to print either"yes" of "no".
Yes, your main should reflect the above description you've been given:
receive a string of characters
checks if this string is a palindrome
print either "yes" of "no"
In other words you might have something like:
int len = input_string();
if ( check_palindrome(len) ) {
print "yes";
else
print "no"
Suggest you write it in C or other language you know, then translate that to assembly.
Also consider that we some things we program are functions returning a value rather than procedures that don't return values. Returning a value so that main can take a different course of action (e.g. print yes vs. no) is much better than using non-local goto's to alter the flow of control from within a subroutine.
If your instruction/coursework has given you that main, and is recommending non-local goto's that would be very sad.
I feel for you and your classmates, as this is one of the worst examples of teaching assembly I've seen in a long long time.
array_ptr: # Label pointing to 100 word array
.space 100
The label name is misleading. This space is used as an array of words, not a pointer to an array. The storage reserved is 25 words, since .space operates in terms of bytes and words are 4 bytes each. So, the comment is just plain wrong.
The various "functions" called using jal are single use function, so there's really no need for functions in this assignment at all. The "functions" also are going to each other and back to main instead of returning properly like they would in structured programming. So, this is what we call spaghetti code — such code is difficult to reason over and one of the reasons that other languages don't even bother to offer this kind of flow control.
The array being used is storing whole words, when the input elements are only characters, so that's harmless but unnecessary.
beq $t6, $t7, convert
convert:
This control structure will never choose between two options, it will always convert. Why? Because in the case $t6 is true it will branch to convert: and in the case that $t6 is not true it will fall through to convert:, so same location, will run same code in either case.
You should be able to observe this during debugging.
Debugging Tips
Get to know your data. You should know the address of the array as you debug. You can find this during execution, e.g. look at a register after li ... array_ptr (btw, that opcode should be la, but no matter if it works). Otherwise you can observe the data section and its layout to find that out before running the first instruction.
Single step each line like one would to debug code in any other language, verifying program state between each line. In MIPS assembly, not much program state changes between lines so usually this is pretty simple — usually each instruction only changes one register or one memory location — but you must verify that such change is as you're expecting. Once the first part of the program is properly storing characters into the array, you can use the break point feature to stop at the palindrome check routine and single step only from there on.
Use the smallest possible input first, (in the most degenerate case that would be an empty string, but you may not be handling those so instead) might try a single letter input (should be a palindrome). Once that is working, try two letter input. As I said, first make sure that the character values are being placed into the array properly, and only when you've verified that's working, go on to debug the palindrome check code.
I need to make a mips program that when given an integer, will print all possible numbers with that number of bits. What is the best was to do this?
This might help get you started. It's a way to count the number of 1s in a binary number.
popcnt:
;input: $a0 = the 32-bit number you wish to check
;output: $v0 = the number of bits that equal 1.
move $v0,$zero
li $t0,32
loop_popcnt:
move $a1,$a0
andi $a1,$a1,1
beqz $a1,skip # if zero, the bit we tested was zero, so don't add 1 to the answer.
nop # branch delay slot. We don't want the next instruction to execute if we branch
addiu $v0,$v0,1
skip:
ror $a0,$a0,1 # next bit
addiu $t0,$t0,-1
bnez $t0,loop_popcnt
nop #branch delay slot.
jr $ra
I just want to check my understanding of these two concepts is correct, as I have been trying to finish a project and while everything works to my expectations, it keeps narrowly failing the test cases and introducing a random value...
Basically, the objective of the project is to write out a branch instruction to console in this form:
BranchName $S, [$t, if applicable] 0xAbsoluteAddressOfBranchTargetInstruction
Edit: Clarification: I'm writing this in MIPS. The idea is I get a memory address in $a0 given to the program by my instructor's code (I write the function). The address is for the word containing a MIPS instruction. I'm to do the following:
Get instruction
Isolate instruction opcode and output its name to register (ie: opcode 5, output BNE), do nothing if it isn't a branch instruction.
Isolate $s, $t, and output as applicable (ie: no $t for bgez)
Use offset in the branch instruction to calculate its absolute address (the address of the target instruction following branch) and output in hex. For the purposes of this calculation, the address of the branch instruction ($a0) is assumed to be $pc.
IE:
BEQ $6, $9, 0x00100008
Firstly, is my understanding of branch calculation correct?
PC -> PC + 4
Lower 16 bits of instruction
<< 2 these lower bits
Add PC+4 and the left shifted lower 16 bits (only the lower 16 though).
Secondly, could somebody tell me which bits I need to isolate to know what kind of branch I'm dealing with? I think I have them (first 6 for BEQ/BNE, first 16 with $s masked out for others) but I wanted to double check.
Oh, and finally... should I expect deviation on SPIM from running it on an Intel x86 Windows system and an Intel x86 Linux system? I'm getting a stupid glitch and I cannot seem to isolate it from my hand-worked address calculations, but it only shows up when I run the test scripts my prof gave us on Linux (.sh); running directly in spim on either OS seems to work... provided my understanding of how to do the hand calculations (as listed above) is correct.
This is prefaced by my various comments.
Here is a sample program that does the address calculation correctly. It does not do the branch instruction type decode, so you'll have to combine parts of this and your version together.
Note that it uses the mars syscall 34 to print values in hex. This isn't available under spim, so you may need to output in decimal using syscall 1 or write your own hex value output function [if you haven't already]
.data
msg_best: .asciiz "correct target address: "
msg_tgt: .asciiz "current target address: "
msg_nl: .asciiz "\n"
.text
.globl main
main:
la $s0,inst # pointer to branch instruction
la $s1,einst # get end of instructions
subu $s1,$s1,$s0 # get number of bytes
srl $s1,$s1,2 # get number of instruction words
la $s2,loop # the correct target address
la $a0,msg_best
move $a1,$s2
jal printaddr
loop:
move $a0,$s0
jal showme # decode and print instruction
addiu $s0,$s0,4
sub $s1,$s1,1
bnez $s1,loop # more to do? yes, loop
li $v0,10
syscall
# branch instructions to decode
inst:
bne $s0,$s1,loop
beq $s0,$s1,loop
beqz $s1,loop
bnez $s1,loop
bgtz $s1,loop
bgez $s1,loop
bltz $s1,loop
blez $s1,loop
einst:
# showme -- decode and print data about instruction
#
# NOTE: this does _not_ decode the instruction type
#
# arguments:
# a0 -- instruction address
#
# registers:
# t5 -- raw instruction word
# t4 -- branch offset
# t3 -- absolute address of branch target
showme:
subu $sp,$sp,4
sw $ra,0($sp)
lw $t5,0($a0) # get inst word
addiu $t3,$a0,4 # get PC + 4
sll $t4,$t5,16 # shift offset left
sra $t4,$t4,16 # shift offset right (sign extend)
sll $t4,$t4,2 # get byte offset
addu $t3,$t3,$t4 # add in offset
# NOTE: as a diagnostic, we could compare t3 against s2 -- it should
# always match
la $a0,msg_tgt
move $a1,$t3
jal printaddr
lw $ra,0($sp)
addu $sp,$sp,4
jr $ra
# printaddr -- print address
#
# arguments:
# a0 -- message
# a1 -- address value
printaddr:
li $v0,4
syscall
# NOTE: only mars supports this syscall
# to use spim, use a syscall number of 1, which outputs in decimal and
# then hand convert
# or write your own hex output function
move $a0,$a1
li $v0,34 # output number in hex (mars _only_)
syscall
la $a0,msg_nl
li $v0,4
syscall
jr $ra
The 16 bit immediate value is sign-extended to 32 bits, then shifted. I don't know if that would affect your program; but, that's the only potential "mistake" I noticed.
I need to implement an instruction in MIPS assembly that jumps to a location stored in a register if its value is non-negative; otherwise, it jumps to a location stored in a second register.
I'm having an issue with how to check for negative values in registers and also need help understanding how to implement this.
Suppose you have in $t1 the test register (the one pointing to the address to jump if its contents is non-negative), and in $t2 the register which will hold the address of the jump if $t1 is negative.
Then, this snippet should do the trick:
bge $t1, $zero, is_positive
jr $t2
is_positive:
jr $t1
The first instruction branches to is_positive if $t0 is non-negative. The instruction at that label jumps to the address given by $t1. If the branch is not taken (i.e. $t0 is negative), then the following instruction is executed which will jump to the address given by $t2.
How to write the following into MIPS instructions?
$t0=$t1
if ($t6<$t7) go to Label.
$t0 is not reserved for zero. $t0 is a temporary register that can store any value. The register $zero is reserved/hard-wired to zero. We would want to "branch" to "Label" if $t6 is "less than" $t7, so use the branch on less than instruction blt. The code would look like:
add $t0,$zero,$t1
blt $t6,$t7,Label
your following rubbish:
$t0=$t1
if ($t6 less than $t7) go to Label
would be converted to MIPS like:
move $t0,$t1 # or use instruction instead (add $t0,$zero,$t1)|(addi $t0,$t1,0)
slt $t2,$t6,$t7 # if $t6less than $t7 set $t2=1
bgtz $t2,foo # if $t2=0 goto foo, and foo is the label that you want to move to
Assuming that the registers are already loaded with the right data.
So for $t2 = $t3, adding $t3 to register zero and storing it in $t2 will work so this is how it would look like :
add $t2,$t3,$t0 - assuming $t0 is reserved for zero like most versions of mips.
for if $t4, we need a branch statement, not sure what you want to compare it to, but look at this guide - should give enough instructions about how to write it.