What Is The Most Efficient Arm Instruction Code To Load A Large Constant 41952 Into Register R0

ARM uses a load-store model for memory access which means that only load/shop (LDR and STR) instructions can access memory. While on x86 almost instructions are immune 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 32-flake value at a particular memory address on ARM would require iii types of instructions (load, increment, and store) to first load the value at a particular accost into a annals, increment information technology inside the register, and shop it dorsum to the memory from the register.

To explain the fundamentals of Load and Shop operations on ARM, we start with a basic example and go along with three basic commencement forms with iii different address modes for each starting time form. For each example we volition use the same piece of assembly code with a different LDR/STR starting time grade, to go along it simple. The best mode to follow this part of the tutorial is to run the code examples in a debugger (GDB) on your lab environment.

Offset form: Firsthand value equally the offset
- Addressing mode: Outset
- Addressing mode: Pre-indexed
- Addressing mode: Mail service-indexed
Showtime form: Register every bit the showtime
- Addressing style: Offset
- Addressing mode: Pre-indexed
- Addressing mode: Post-indexed
Offset form: Scaled register every bit the starting time
- Addressing fashion: Outset
- Addressing manner: Pre-indexed
- Addressing mode: Post-indexed

First basic example

By and large, 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.

LDR R2, [R0]   @ [R0] - origin address is the value found in R0. STR R2, [R1]   @ [R1] - destination address is the value found in R1.

LDR operation: loads thevalue at the address found in R0 to the destination register R2.

STR functioning: stores the value found in R2 to the memory address establish in R1.

This is how it would look similar in a functional assembly program:

.data          /* the .information section is dynamically created and its addresses cannot be hands predicted */ var1: .give-and-take iii  /* variable 1 in memory */ var2: .discussion 4  /* variable 2 in memory */  .text          /* start of the text (code) section */  .global _start  _start:     ldr r0, adr_var1  @ load the retentivity accost of var1 via label adr_var1 into R0      ldr r1, adr_var2  @ load the retention address of var2 via label adr_var2 into R1      ldr r2, [r0]      @ load the value (0x03) at retentivity address constitute in R0 to register R2       str r2, [r1]      @ shop the value institute in R2 (0x03) to the memory address establish in R1      bkpt               adr_var1: .word var1  /* address to var1 stored hither */ adr_var2: .discussion var2  /* address to var2 stored hither */

At the bottom we have our Literal Pool (a memory surface area in the same code section to shop constants, strings, or offsets that others can reference in a position-independent mode) where nosotros store the retention addresses of var1 and var2 (divers in the data section at the tiptop) using the labels adr_var1 and adr_var2. The first LDR loads the address of var1 into register R0. The second LDR does the aforementioned for var2 and loads information technology to R1. Then we load the value stored at the memory address institute in R0 to R2, and shop the value found in R2 to the memory accost constitute in R1.

When we load something into a annals, the brackets ([ ]) mean: the value found in the register between these brackets is a memory accost we desire to load something from.

When nosotros store something to a memory location, the brackets ([ ]) mean: the value found in the register between these brackets is a retentivity address we want to store something to.

This sounds more complicated than it actually is, and then here is a visual representation of what's going on with the retentiveness and the registers when executing the code in a higher place in a debugger:

Let'southward expect at the same code in a debugger.

                      gef>                    disassemble _start Dump of assembler code for function _start:  0x00008074 <+0>:      ldr  r0,          [pc, #12]          ; 0x8088 <adr_var1>  0x00008078 <+4>:      ldr  r1,          [pc, #12]          ; 0x808c <adr_var2>  0x0000807c <+8>:      ldr  r2, [r0]  0x00008080 <+12>:     str  r2, [r1]  0x00008084 <+16>:     bx   lr End of assembler dump.

The labels we specified with the first two LDR operations changed to [pc, #12]. This is called PC-relative addressing. Because we used labels, the compiler calculated the location of our values specified in the Literal Pool (PC+12). Yous can either calculate the location yourself using this exact approach, or you can use labels similar we did previously. The merely deviation is that instead of using labels, yous need to count the exact position of your value in the Literal Pool. In this example, information technology is 3 hops (4+4+four=12) abroad from the effective PC position. More virtually PC-relative addressing afterward in this chapter.

Side annotation: In case you forgot why the effective PC is located two instructions ahead of the current one, it is described in Part 2 [… During execution, PC stores the address of the electric current instruction plus viii (ii ARM instructions) in ARM land, and the current instruction plus 4 (ii Pollex instructions) in Thumb state. This is different from x86 where PC ever points to the next instruction to be executed…].

1.Kickoff form: Firsthand value every bit the first

STR    Ra, [Rb,          imm] LDR    Ra, [Rc,          imm]

Here we utilize an immediate (integer) as an kickoff. This value is added or subtracted from the base of operations register (R1 in the example beneath) to access information at an offset known at compile time.

.data var1: .word 3 var2: .discussion four  .text .global _start  _start:     ldr r0, adr_var1  @ load the retention address of var1 via label adr_var1 into R0     ldr r1, adr_var2  @ load the memory address of var2 via label adr_var2 into R1     ldr r2, [r0]      @ load the value (0x03) at memory address institute in R0 to register R2                      str r2, [r1, #2]                      @ address manner: offset. Store the value establish in R2 (0x03) to the retention accost found in          R1 plus ii. Base register (R1) unmodified.                                                    str r2, [r1, #4]!                      @ address mode: pre-indexed. Store the value institute in R2 (0x03) to the memory accost found in          R1 plus 4. Base of operations register (R1) modified: R1 = R1+4                                    ldr r3, [r1]            , #4                    @ address style: mail-indexed. Load the value at retention address found in R1 to register          R3. Base register (R1) modified: R1 = R1+4      bkpt  adr_var1: .give-and-take var1 adr_var2: .word var2

Permit'southward phone call this program ldr.southward, compile it and run it in GDB to meet what happens.

$ equally ldr.s -o ldr.o $ ld ldr.o -o ldr $ gdb ldr

In GDB (with gef) we prepare a break signal at _start and run the program.

                      gef>                    break _start                      gef>                    run ...                      gef>                    nexti iii     /* to run the next 3 instructions */

The registers on my system are now filled with the following values (keep in mind that these addresses might be different on your system):

$r0 :          0x00010098          -> 0x00000003 $r1 :          0x0001009c          -> 0x00000004 $r2 :          0x00000003          $r3 : 0x00000000 $r4 : 0x00000000 $r5 : 0x00000000 $r6 : 0x00000000 $r7 : 0x00000000 $r8 : 0x00000000 $r9 : 0x00000000 $r10 : 0x00000000 $r11 : 0x00000000 $r12 : 0x00000000 $sp : 0xbefff7e0 -> 0x00000001 $lr : 0x00000000 $pc : 0x00010080 -> <_start+12> str r2, [r1] $cpsr : 0x00000010

The side by side education that will be executed a STR performance with the offset address mode . Information technology will store the value from R2 (0x00000003) to the memory address specified in R1 (0x0001009c) + the offset (#2) = 0x1009e.

                      gef>                    nexti                      gef>                      10/west 0x1009e  0x1009e <var2+two>: 0x3

The side by side STR operation uses the pre-indexed address manner . You tin recognize this mode by the exclamation marking (!). The just difference is that the base register volition be updated with the last retentivity address in which the value of R2 will be stored. This means, we shop the value found in R2 (0x3) to the memory address specified in R1 (0x1009c) + the beginning (#4) = 0x100A0, and update R1 with this exact address.

                      gef>                      nexti                          gef>                    x/w 0x100A0 0x100a0: 0x3                      gef>                    info register r1 r1          0x100a0          65696

The concluding LDR functioning uses the postal service-indexed accost mode . This means that the base annals (R1) is used as the last accost, then updated with the offset calculated with R1+four. In other words, information technology takes the value plant in R1 (non R1+4), which is 0x100A0 and loads information technology into R3, then updates R1 to R1 (0x100A0) + offset (#4) = 0x100a4.

                      global environment facility>                    info annals r1 r1          0x100a4          65700                      gef>                    info register r3 r3          0x3          3

Here is an abstruse illustration of what'southward happening:

two.Offset form: Annals as the outset.

STR    Ra, [Rb,          Rc] LDR    Ra, [Rb,          Rc]

This starting time form uses a register equally an offset. An example usage of this offset form is when your code wants to admission an array where the index is computed at run-fourth dimension.

.data var1: .word iii var2: .word 4  .text .global _start  _start:     ldr r0, adr_var1  @ load the memory address of var1 via label adr_var1 to R0      ldr r1, adr_var2  @ load the retentivity accost of var2 via characterization adr_var2 to R1      ldr r2, [r0]      @ load the value (0x03) at memory accost found in R0 to R2                                    str r2, [r1, r2]                      @ accost mode: offset. Store the value plant in R2 (0x03) to the memory address constitute in R1 with the offset R2 (0x03). Base register unmodified.                                    str r2, [r1, r2]!                      @ address fashion: pre-indexed. Store value found in R2 (0x03) to the memory address plant in R1 with the first R2 (0x03). Base register modified: R1 = R1+R2.                      ldr r3, [r1], r2                                @ address style: post-indexed. Load value at memory address institute in R1 to annals R3. And so alter base of operations register: R1 = R1+R2.     bx lr  adr_var1: .word var1 adr_var2: .word var2

Later executing the get-go STR performance with the start address mode , the value of R2 (0x00000003) will exist stored at retentiveness address 0x0001009c + 0x00000003 = 0x0001009F.

                      gef>                    x/westward 0x0001009F  0x1009f <var2+3>: 0x00000003

The second STR operation with the pre-indexed address style will practise the aforementioned, with the difference that it volition update the base annals (R1) with the calculated retention address (R1+R2).

                      gef>                    info register r1  r10x1009f          65695

The last LDR operation uses the post-indexed accost mode and loads the value at the memory address found in R1 into the register R2, then updates the base register R1 (R1+R2 = 0x1009f + 0x3 = 0x100a2).

                      gef>                    info register r1  r1          0x100a2          65698                      gef>                    info register r3  r30x3          3

3.Offset course: Scaled register as the beginning

LDR    Ra, [Rb,          Rc, <shifter>] STR    Ra, [Rb,          Rc, <shifter>]

The third offset form has a scaled register equally the commencement. In this case, Rb is the base of operations annals and Rc is an firsthand offset (or a register containing an firsthand value) left/right shifted (<shifter>) to calibration the immediate. This means that the barrel shifter is used to calibration the offset. An example usage of this offset form would be for loops to iterate over an array. Here is a simple example you lot can run in GDB:

.information var1: .discussion 3 var2: .word 4  .text .global _start  _start:     ldr r0, adr_var1         @ load the retentiveness address of var1 via label adr_var1 to R0     ldr r1, adr_var2         @ load the memory accost of var2 via label adr_var2 to R1     ldr r2, [r0]             @ load the value (0x03) at memory address found in R0 to R2                      str r2, [r1, r2, LSL#ii]                                @ accost manner: starting time. Store the value found in R2 (0x03) to the memory address found in R1 with the offset R2 left-shifted by ii. Base register (R1) unmodified.                      str r2, [r1, r2, LSL#2]!                      @ address mode: pre-indexed. Store the value constitute in R2 (0x03) to the retentivity address institute in R1 with the first R2 left-shifted by 2. Base register modified: R1 = R1 + R2<<ii                                    ldr r3, [r1], r2, LSL#2                                @ address fashion: postal service-indexed. Load value at memory address found in R1 to the annals R3. So modifiy base of operations register: R1 = R1 + R2<<2     bkpt  adr_var1: .give-and-take var1 adr_var2: .word var2

The first STR operation uses the offset accost mode and stores the value plant in R2 at the retentiveness location calculated from [r1, r2, LSL#two], which ways that information technology takes the value in R1 every bit a base (in this example, R1 contains the retentiveness address of var2), and then information technology takes the value in R2 (0x3), and shifts it left by two. The picture below is an attempt to visualize how the memory location is calculated with [r1, r2, LSL#two].

The second STR operation uses the pre-indexed accost mode . This means, it performs the same action as the previous operation, with the departure that it updates the base register R1 with the calculated retention accost afterwards. In other words, it will first shop the value found at the memory accost R1 (0x1009c) + the commencement left shifted by #2 (0x03 LSL#2 = 0xC) = 0x100a8, and update R1 with 0x100a8.

                      gef>                    info register r1 r1           0x100a8          65704

The last LDR operation uses the post-indexed address mode . This means, information technology loads the value at the retention address constitute in R1 (0x100a8) into register R3, and so updates the base register R1 with the value calculated with r2, LSL#2. In other words, R1 gets updated with the value R1 (0x100a8) + the kickoff R2 (0x3) left shifted by #2 (0xC) = 0x100b4.

                      gef>                    info annals r1 r10x100b4          65716

Summary

Recall the three offset modes in LDR/STR:

offset style uses an immediate as offset
- ldr r3, [r1, #4]
offset fashion uses a register as offset
- ldr r3, [r1, r2]
beginning mode uses a scaled register as offset
- ldr r3, [r1, r2, LSL#2]

How to remember the different address modes in LDR/STR:

If at that place is a !, it's prefix address mode
- ldr r3, [r1, #iv]!
- ldr r3, [r1, r2]!
- ldr r3, [r1, r2, LSL#ii]!
If the base annals is in brackets past itself, information technology's postfix address mode
- ldr r3, [r1], #4
- ldr r3, [r1], r2
- ldr r3, [r1], r2, LSL#2
Anything else is outset address style.
- ldr r3, [r1, #4]
- ldr r3, [r1, r2]
- ldr r3, [r1, r2, LSL#2]

LDR is non only used to load data from memory into a register. Sometimes you lot volition run into syntax like this:

.department .text .global _start  _start:    ldr r0, =leap        /* load the address of the function label bound into R0 */    ldr r1, =0x68DB00AD  /* load the value 0x68DB00AD into R1 */ jump:    ldr r2, =511         /* load the value 511 into R2 */     bkpt

These instructions are technically called pseudo-instructions. We tin can use this syntax to reference information in the literal pool. The literal pool is a memory area in the same section (because the literal puddle is part of the code) to store constants, strings, or offsets. In the example above we use these pseudo-instructions to reference an first to a function, and to movement a 32-bit abiding into a register in one instruction. The reason why we sometimes need to utilize this syntax to motion a 32-flake constant into a register in 1 instruction is because ARM can only load a 8-chip value in one go. What? To understand why, y'all need to know how immediate values are being handled on ARM.

Loading firsthand values in a register on ARM is non as straightforward every bit it is on x86. In that location are restrictions on which firsthand values you can use. What these restrictions are and how to bargain with them isn't the nearly exciting role of ARM assembly, but bear with me, this is just for your understanding and in that location are tricks you can employ to bypass these restrictions (hint: LDR).

We know that each ARM instruction is 32bit long, and all instructions are conditional. There are sixteen status codes which we can use and one condition lawmaking takes up four $.25 of the instruction. Then we demand 2 bits for the destination register. 2 bits for the first operand register, and 1 bit for the set-status flag, plus an contrasted number of bits for other matters similar the bodily opcodes. The bespeak here is, that after assigning bits to instruction-blazon, registers, and other fields, there are only 12 $.25 left for immediate values, which will but allow for 4096 different values.

This means that the ARM teaching is only able to employ a limited range of immediate values with MOV directly. If a number tin't be used directly, it must exist separate into parts and pieced together from multiple smaller numbers.

But in that location is more. Instead of taking the 12 $.25 for a single integer, those 12 bits are split up into an 8bit number (n) being able to load any 8-fleck value in the range of 0-255, and a 4bit rotation field (r) being a right rotate in steps of two between 0 and 30. This means that the full immediate value v is given by the formula: v = due north ror ii*r. In other words, the only valid immediate values are rotated bytes (values that tin can be reduced to a byte rotated by an even number).

Here are some examples of valid and invalid immediate values:

Valid values: #256        // 1 ror 24 --> 256 #384        // half-dozen ror 26 --> 384 #484        // 121 ror 30 --> 484 #16384      // i ror 18 --> 16384 #2030043136 // 121 ror eight --> 2030043136 #0x06000000 // half dozen ror 8 --> 100663296 (0x06000000 in hex)  Invalid values: #370        // 185 ror          31 -->                      31 is not in range (0 – 30)          #511        // 1 1111 111ane          --> bit-design can't fit into one byte #0x06010000 // 1 1000 0001.. --> bit-design can't fit into one byte

This has the consequence that it is not possible to load a full 32bit address in one become. Nosotros tin bypass this restrictions past using one of the following ii options:

Construct a larger value out of smaller parts
1. Instead of using MOV r0, #511
2. Carve up 511 into two parts: MOV r0, #256, and ADD r0, #255
Use a load construct 'ldr r1,=value' which the assembler volition happily convert into a MOV, or a PC-relative load if that is not possible.
1. LDR r1, =511

If yous try to load an invalid firsthand value the assembler will complain and output an mistake saying: Error: invalid constant. If you lot run across this fault, you now know what it means and what to practise nigh it.
Let's say you want to load #511 into R0.

.department .text .global _start  _start:     mov     r0, #511     bkpt

If you lot endeavor to assemble this code, the assembler will throw an error:

azeria@labs:~$ as test.southward -o test.o test.s: Assembler messages: test.due south:5: Error: invalid abiding (1ff) after fixup

You lot need to either dissever 511 in multiple parts or y'all apply LDR as I described before.

.section .text .global _start  _start:  mov r0, #256   /* 1 ror 24 = 256, so it's valid */  add r0, #255   /* 255 ror 0 = 255, valid. r0 = 256 + 255 = 511 */  ldr r1, =511   /* load 511 from the literal pool using LDR */  bkpt

If you demand to figure out if a certain number can be used as a valid immediate value, y'all don't need to summate it yourself. Y'all can employ my petty python script called rotator.py which takes your number equally an input and tells yous if it can be used as a valid immediate number.

azeria@labs:~$ python rotator.py Enter the value you desire to check: 511  Distressing, 511 cannot be used equally an firsthand number and has to be separate.  azeria@labs:~$ python rotator.py Enter the value you lot want to bank check: 256  The number 256 can be used as a valid immediate number. 1 ror 24 --> 256