We have already used variable length strings in our previous examples. The variable length strings can have as many characters as required. We can also write. Alternatively, you can store strings with a trailing sentinel character to delimit a string instead of storing the string length explicitly. The sentinel character should be a special character that does not appear within a string. Each string instruction may require a source operand, a destination operand or both. For bit segments, string instructions use ESI and EDI registers to point to the source and destination operands, respectively.
For bit segments, however, the SI and the DI registers are used to point to the source and destination, respectively. If the operand is of one byte, it is loaded into the AL register, if the operand is one word, it is loaded into the AX register and a doubleword is loaded into the EAX register.
Data could be of a byte size, word or doubleword. Each of the above instruction has a byte, word, and doubleword version, and string instructions can be repeated by using a repetition prefix. The following table provides various versions of string instructions and the assumed space of the operands.
It repeats the instruction processing until CX is zero. Assembly - Strings Advertisements. Previous Page. Next Page. Previous Page Print Page. Dashboard Logout.This document is not intended as a beginner's guide to MIPS. It is intended for people that have coded some with MIPS and feel somewhat comfortable with its use. If this is not you you will not get much out of this document. This document provides examples that are supposed to give greater insight into what MIPS does, and how to use MIPS for more or less useful applications.
I cover how to read in strings in MIPS and what happens to memory when you read in strings. I also cover using arrays in MIPS. I will provide a very simple example to give a feel for syscall functionality for reading in strings. I provide a copy of the table here. The "Arguments" column explains what should be in a specific argument register or registers before a specific syscall. The "Result" column tells what the contents of registers will hold after the syscall.
If you want to read an int, you simply do a syscall 5. This is not correct. How could, say, a byte string fit into a 4 byte quantity? That doesn't make sense. The next three lines of "la" and "li" statements set registers to appropriate values before we say "syscall". This is where you should take look at the table at the top of A I provide a line of the code, and then some background.
When you call "syscall" in your code, a value called the "system call code" will determine what function syscall performs. I chose 64 characters. You can have it be 50, oror 37, or whatever you like, but you shouldn't go above 64 in this example because in the first part of this program you only set aside 64 bytes using the ".
Upon receiving the syscall command, the system says, "what do I need to do? Each block represents a byte in data.
The blocks are adjacent, and so are the bytes in memory. The first byte is the byte referenced by "theString", and the string is termined by a null character.
After syscall is finished, the byte referenced by "theString" would contain the ascii value for 'H', the next byte would contain 'e', etc, etc. For an explanation of "vectors" in SPIM, I will construct a SPIM program that calculates the first 40 terms of my favorite sequence, the Fibonacci sequence, and stores it in an array like structure. First we see an effort in C. The intention for providing this code is to create a natural flow from C to MIPS, and to demonstrate how arrays in C and arrays in MIPS need not be thought of as radically different entities.
I assume familiarity with C, and some basic familiarity with how to read data to and from memory in MIPS specifically with lw and sw. Incidentally, my work that follows is often purposefully inefficient for the purpose of greater clarity, though sometimes being clear one way leads to being unclear in some other way.
I wrote this all very late at night while afflicted with insomnia. Presumably we're all familiar with C, so we can see how this program works. Just look over it until it begins to make sense, because aside from the ambiguous variable names it's not that tough.
We then "translate" this into MIPS assembler language. When we do so, there are several differences that must be kept in mind. The thing with "arrays" in MIPS, if we're to call them that, is that the "indices" are always incremented in terms of bytes.While on x86 most instructions are allowed to directly operate on data in memory, on ARM data must be moved from memory into registers before being operated on.
This means that incrementing a bit value at a particular memory address on ARM would require three types of instructions load, increment, and store to first load the value at a particular address into a register, increment it within the register, and store it back to the memory from the register.
The best way to follow this part of the tutorial is to run the code examples in a debugger GDB on your lab environment. Generally, LDR is used to load something from memory into a register, and STR is used to store something from a register to a memory address.
The first LDR loads the address of var1 into register R0. The second LDR does the same for var2 and loads it to R1. Then we load the value stored at the memory address found in R0 to R2, and store the value found in R2 to the memory address found in R1.
When we load something into a register, the brackets [ ] mean: the value found in the register between these brackets is a memory address we want to load something from. The labels we specified with the first two LDR operations changed to [pc, 12]. This is called PC-relative addressing. You can either calculate the location yourself using this exact approach, or you can use labels like we did previously.
The only difference is that instead of using labels, you need to count the exact position of your value in the Literal Pool. More about PC-relative addressing later in this chapter. This is different from x86 where PC always points to the next instruction to be executed…]. Here we use an immediate integer as an offset. This value is added or subtracted from the base register R1 in the example below to access data at an offset known at compile time.
The registers on my system are now filled with the following values keep in mind that these addresses might be different on your system :. The next STR operation uses the pre-indexed address mode. You can recognize this mode by the exclamation mark! The only difference is that the base register will be updated with the final memory address in which the value of R2 will be stored.
The last LDR operation uses the post-indexed address mode. This offset form uses a register as an offset. The third offset form has a scaled register as the offset.C to MIPS - Introduction to Memory
This means that the barrel shifter is used to scale the offset.I remember my college days when i was really having trouble with my Java subject.
Hi there! Plz reply me at mnpham csupomona. Thank you for visiting my blog and commenting my posts. If I increase the address by 4 I won't get the next byte [ I'm using lb load byte above, not lw load word ] but the first byte of the next word. As I think I explain above the code you cannot reverse the 1st word with the last one and get the correct string reversed.
You must reverse it byte-byte char-char to have correct result. So I must increase index by 1 and not 4. I hope this will help you understand this example and may smt more. If you still don't get smt I 'm expecting your next comment!
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MIPS Instruction Set
JAVA and C programming examples and advices. Tips about computers. Few MIPS examples and advices about assembly. Thank you for visiting. Search This Blog. MIPS example to reverse a string.
First it will be better to show how MIPS stores strings. Newer Post Older Post Home. Subscribe to: Post Comments Atom. Reality Software.Skip to content. Skip to navigation. For class, you should use the register names, not the corresponding register numbers. Remainder stored in special register hi Quotient stored in special register lo. All conditional branch instructions compare the values in two registers together.
If the comparison test is true, the branch is taken i. Otherwise, the processor continues on to the next instruction. Note 1: It is much easier to use a label for the branch instructions instead of an absolute number. The label "equal" should be defined somewhere else in the code.
Note 2: There are many variations of the above instructions that will simplify writing programs! Note: It is much easier to use a label for the jump instructions instead of an absolute number. That label should be defined somewhere else in the code. The SPIM simulator provides a number of useful system calls. These are simulatedand do not represent MIPS processor instructi ons. System calls are used for input and output, and to exit the program. They are initiated by the syscall instruction.
In other words, not all registers are used by all system calls. An assembler directive allows you to request the assembler to do something when converting your source code to binary code. MIPS has 32 general-purpose registers that could, technically, be used in any manner the programmer desires.
However, by convention, registers have been divided into groups and used for different purposes. Registers have both a number used by the hardware and a name used by the assembly programmer.
Skip to navigation Personal tools Log in. Search Site only in current section. Advanced Search…. Lower 16 bits are set to zero.
Copy from register to register. Upper 32 bits stored in special register hi Lower 32 bits stored in special register lo. Pseudo-instruction provided by assembler, not processor!
Loads computed address of label not its contents into register. Loads immediate value into register. Returns the address to a block of memory containing n additional bytes.
Useful for dynamic memory allocation. Strings are in double-quotes, i. For example.Tag: assemblymipsspim. I have to write a program in assembly language that is to store in memory the first integers from 0 to You don't have any code to break out of your loop, so you'll just keep on writing bytes forever, which eventually will result in a Bad Address exception. You need to add some instructions in your loop that check if you have written bytes, and if so doesn't jump back to loop:.
Another problem is that you've written the operands for sb in the wrong order. The problem is, you don't have debug info for the ptr type, so gdb treats it as integer. Somewhat ironically, by using UAL syntax to solve the first problem you've now hit pretty much the same thing, but the other way round and with a rather more cryptic symptom.
The only Thumb encodings for non-flag-setting mov with an immediate operand are bit ones, however Cortex-M0 doesn't support those, Your first instruction is the problem: cmp rdi, 0. I'm supposing if you pass a 0 pointer, that would be But, you I am guessing that you want nome to be an array of integers. The lodsb instruction loads the byte pointed to by the DS and SI registers but you haven't loaded either with a valid value.
Since this a bootloader you also need to use the ORG directive, otherwise the assembler won't know where you code, and therefor welcome, gets loaded into memory Here's an example from a course that I teach. This makes use of the real mode The immediate problem is that your prints destroys bx because it sets bl and bh so your printmem loop which requires bx to be preserved blows up. However, it also destroys al so your input loop won't be storing the correct value in memory to start with, either.
Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I've successfully stored the ID and Titles in memory dynamically, but I'm having trouble storing the user-entered strings.
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This is a complex question and there's not a lot of info that I've been able to find online so props if you can help me out with this :. The problem is that all that's being stored is the address of the buffers. The buffers are defined in my data section like this:. What I end up with is just the addresses stored in the memory I allocated, and I have no idea how to store strings into allocated memory.
Don't bother defining your memory at the start of your program like I showed in the original question. That will store ID, Year, Title, and Description - the offset is where we place the data in the dynamic memory.
Learn more. Ask Question. Asked 8 years, 6 months ago. Active 2 years, 7 months ago. Viewed 14k times. Okay guys, basically the problem I'm having is this. I've been assigned to write a MIPS program that stores a struct dynamically. Any help would be greatly appreciated! Rustam A. Gasanov 13k 6 6 gold badges 52 52 silver badges 70 70 bronze badges. Active Oldest Votes. Instead, allocate it and read the values into the correct offsets of your dynamic memory.
Imagine that I have a block of bytes like above : [ ] We store the ID an integer 4 bytes of data at offset 0 like this: [ ID ] Then, we store the year next to it at offset 4. Sign up or log in Sign up using Google.