How To Increment A File Register To 10 Using A Loop Assembly

ARM uses a load-store model for retention access which ways that simply load/shop (LDR and STR) instructions can admission memory. While on x86 most instructions are allowed to directly operate on data in memory, on ARM information must be moved from memory into registers before being operated on. This means that incrementing a 32-bit value at a detail 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, increase information technology inside the register, and store it back to the retention from the register.

To explain the fundamentals of Load and Store operations on ARM, we outset with a basic instance and go on with three basic get-go forms with three unlike address modes for each offset form. For each example nosotros will use the same piece of assembly code with a unlike LDR/STR offset form, to keep it simple. The best fashion to follow this function of the tutorial is to run the code examples in a debugger (GDB) on your lab environment.

1. Get-go form: Firsthand value as the offset
   • Addressing mode: Offset
   • Addressing style: Pre-indexed
   • Addressing fashion: Post-indexed
2. Offset grade: Register as the offset
   • Addressing way: Showtime
   • Addressing mode: Pre-indexed
   • Addressing mode: Post-indexed
3. Get-go form: Scaled register as the offset
   • Addressing way: Offset
   • Addressing style: Pre-indexed
   • Addressing style: Post-indexed

First basic case

Generally, LDR is used to load something from retentiveness into a register, and STR is used to store something from a register to a memory accost.

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

LDR performance: loads thevalue at the address institute in R0 to the destination annals R2.

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

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

.data          /* the .data section is dynamically created and its addresses cannot be easily predicted */ var1: .word 3  /* variable 1 in memory */ var2: .word 4  /* variable 2 in retentivity */  .text          /* start of the text (code) section */  .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 accost of var2 via label adr_var2 into R1      ldr r2, [r0]      @ load the value (0x03) at memory accost found in R0 to register R2       str r2, [r1]      @ store the value found in R2 (0x03) to the retentivity address found in R1      bkpt               adr_var1: .word var1  /* accost to var1 stored here */ adr_var2: .word var2  /* address to var2 stored here */

At the bottom we accept our Literal Pool (a memory area in the same lawmaking section to store constants, strings, or offsets that others tin reference in a position-contained manner) where we store the retention addresses of var1 and var2 (divers in the data department at the top) using the labels adr_var1 and adr_var2. The first LDR loads the accost 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 retentivity accost found in R0 to R2, and store the value found in R2 to the retention address found in R1.

When nosotros 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.

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

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

Let'south look at the same lawmaking in a debugger.

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

The labels we specified with the first two LDR operations inverse to [pc, #12]. This is chosen PC-relative addressing. Because we used labels, the compiler calculated the location of our values specified in the Literal Pool (PC+12).  You can either calculate the location yourself using this exact approach, or you lot tin can use labels like we did previously. The only difference is that instead of using labels, y'all need to count the verbal position of your value in the Literal Pool. In this instance, information technology is 3 hops (4+4+iv=12) abroad from the constructive PC position. More about PC-relative addressing afterwards in this chapter.

Side notation: In case you forgot why the constructive PC is located two instructions ahead of the current 1, it is described in Part ii [… During execution, PC stores the accost of the current teaching plus viii (two ARM instructions) in ARM state, and the current instruction plus 4 (2 Thumb instructions) in Thumb state. This is unlike from x86 where PC ever points to the next educational activity to be executed…].

1.Get-go form: Immediate value as the get-go

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

Here nosotros 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 beginning known at compile time.

.data var1: .word three var2: .word 4  .text .global _start  _start:     ldr r0, adr_var1  @ load the memory address of var1 via characterization 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 accost found in R0 to register R2                      str r2, [r1, #2]                      @ address mode: get-go. Store the value establish in R2 (0x03) to the memory address found in          R1 plus 2. Base register (R1) unmodified.                                                    str r2, [r1, #4]!                      @ accost mode: pre-indexed. Store the value constitute in R2 (0x03) to the memory address found in          R1 plus 4. Base annals (R1) modified: R1 = R1+4                                    ldr r3, [r1]            , #4                    @ address style: mail-indexed. Load the value at memory address plant in R1 to register          R3. Base register (R1) modified: R1 = R1+4      bkpt  adr_var1: .word var1 adr_var2: .give-and-take var2

Let's phone call this program ldr.due south, compile it and run it in GDB to see what happens.

$ as ldr.due south -o ldr.o $ ld ldr.o -o ldr $ gdb ldr

In GDB (with gef) we set a break point at _start and run the plan.

                      gef>                    break _start                      gef>                    run ...                      global environment facility>                    nexti 3     /* to run the next 3 instructions */

The registers on my system are at present filled with the following values (continue in mind that these addresses might be unlike 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 next instruction that will exist executed a STR performance with the offset accost mode . It will store the value from R2 (0x00000003) to the retention accost specified in R1 (0x0001009c) + the showtime (#2) = 0x1009e.

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

The next STR functioning uses the pre-indexed address manner . Yous tin recognize this manner by the exclamation marker (!). The only difference is that the base register volition exist updated with the final memory address in which the value of R2 will be stored. This means, we store the value found in R2 (0x3) to the memory address specified in R1 (0x1009c) + the offset (#four) = 0x100A0, and update R1 with this verbal address.

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

The last LDR functioning uses the post-indexed address fashion . This means that the base register (R1) is used equally the terminal accost, then updated with the first calculated with R1+4. In other words, it takes the value establish in R1 (non R1+iv), which is 0x100A0 and loads information technology into R3, so updates R1 to R1 (0x100A0) + get-go (#4) =  0x100a4.

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

Here is an abstruse illustration of what's happening:

two.Start grade: Register as the start.

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

This get-go form uses a register every bit an offset. An instance usage of this offset class is when your lawmaking wants to access an assortment where the index is computed at run-time.

.data var1: .discussion 3 var2: .word 4  .text .global _start  _start:     ldr r0, adr_var1  @ load the memory accost of var1 via label adr_var1 to R0      ldr r1, adr_var2  @ load the memory address of var2 via label adr_var2 to R1      ldr r2, [r0]      @ load the value (0x03) at memory accost found in R0 to R2                                    str r2, [r1, r2]                      @ address mode: offset. Shop the value establish in R2 (0x03) to the memory accost institute 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 found in R1 with the offset R2 (0x03). Base register modified: R1 = R1+R2.                      ldr r3, [r1], r2                                @ address style: post-indexed. Load value at memory accost found in R1 to register R3. Then alter base register: R1 = R1+R2.     bx lr  adr_var1: .give-and-take var1 adr_var2: .word var2

After executing the kickoff STR performance with the offset address mode , the value of R2 (0x00000003) will be stored at memory address 0x0001009c + 0x00000003 = 0x0001009F.

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

The second STR functioning with the pre-indexed address mode will exercise the same, with the difference that it volition update the base register (R1) with the calculated memory address (R1+R2).

                      gef>                    info register r1  r10x1009f          65695

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

                      gef>                    info register r1  r1          0x100a2          65698                      global environment facility>                    info register r3  r30x3          3

3.Starting time course: Scaled annals every bit the beginning

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

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

.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 retention address of var2 via label adr_var2 to R1     ldr r2, [r0]             @ load the value (0x03) at retention accost found in R0 to R2                      str r2, [r1, r2, LSL#ii]                                @ accost mode: beginning. Store the value found in R2 (0x03) to the memory address plant in R1 with the offset R2 left-shifted past 2. Base of operations annals (R1) unmodified.                      str r2, [r1, r2, LSL#2]!                      @ accost fashion: pre-indexed. Store the value constitute in R2 (0x03) to the retentiveness address constitute in R1 with the first R2 left-shifted by 2. Base register modified: R1 = R1 + R2<<2                                    ldr r3, [r1], r2, LSL#2                                @ address fashion: mail-indexed. Load value at memory address found in R1 to the register R3. Then modifiy base of operations register: R1 = R1 + R2<<2     bkpt  adr_var1: .word var1 adr_var2: .give-and-take var2

The first STR operation uses the offset address mode and stores the value establish in R2 at the retention location calculated from [r1, r2, LSL#2], which means that it takes the value in R1 as a base (in this case, R1 contains the retention address of var2), then it takes the value in R2 (0x3), and shifts it left past 2. The picture below is an attempt to visualize how the retentiveness location is calculated with [r1, r2, LSL#2].

The 2nd STR operation uses the pre-indexed address mode . This means, information technology performs the same activeness as the previous operation, with the departure that information technology updates the base annals R1 with the calculated retentiveness accost afterwards. In other words, it will get-go store the value plant at the memory address R1 (0x1009c) + the offset left shifted by #2 (0x03 LSL#2 = 0xC) = 0x100a8, and update R1 with 0x100a8.

                      global environment facility>                    info annals r1 r1           0x100a8          65704

The concluding LDR operation uses the post-indexed address way . This means, information technology loads the value at the memory address found in R1 (0x100a8) into register R3, so updates the base of operations annals R1 with the value calculated with r2, LSL#two. In other words, R1 gets updated with the value R1 (0x100a8) + the offset R2 (0x3) left shifted past #ii (0xC) = 0x100b4.

                      gef>                    info register r1 r10x100b4          65716

Summary

Recollect the three offset modes in LDR/STR:

1. offset style uses an immediate as beginning
   • ldr   r3, [r1, #4]
2. offset mode uses a register as starting time
   • ldr   r3, [r1, r2]
3. starting time mode uses a scaled register as beginning
   • ldr   r3, [r1, r2, LSL#ii]

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

• If there is a !, it's prefix address mode
  • ldr   r3, [r1, #4]!
  • ldr   r3, [r1, r2]!
  • ldr   r3, [r1, r2, LSL#ii]!
• If the base of operations register is in brackets by itself, it's postfix accost way
  • ldr   r3, [r1], #4
  • ldr   r3, [r1], r2
  • ldr   r3, [r1], r2, LSL#2
• Annihilation else is starting time address mode.
  • ldr   r3, [r1, #4]
  • ldr   r3, [r1, r2]
  • ldr   r3, [r1, r2, LSL#2]

LDR is non only used to load information from retentiveness into a register. Sometimes you lot will see syntax like this:

.department .text .global _start  _start:    ldr r0, =jump        /* load the address of the part 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 can use this syntax to reference data in the literal pool. The literal pool is a retentiveness area in the aforementioned department (considering the literal pool is part of the code) to store constants, strings, or offsets. In the example above we use these pseudo-instructions to reference an offset to a function, and to move a 32-fleck abiding into a register in one teaching. The reason why we sometimes need to use this syntax to move a 32-bit constant into a register in 1 instruction is because ARM can only load a 8-bit value in one go. What? To sympathize why, you need to know how firsthand values are being handled on ARM.

Loading firsthand values in a register on ARM is not as straightforward every bit it is on x86. There are restrictions on which firsthand values you can utilize. What these restrictions are and how to deal with them isn't the most heady part of ARM associates, but bear with me, this is just for your agreement and in that location are tricks you can use to bypass these restrictions (hint: LDR).

Nosotros know that each ARM instruction is 32bit long, and all instructions are conditional. At that place are 16 condition codes which we can apply and one condition lawmaking takes up 4 bits of the instruction. And then we need 2 bits for the destination annals. 2 bits for the first operand annals, and 1 flake for the set-condition flag, plus an assorted number of bits for other matters like the actual opcodes. The bespeak here is, that after assigning $.25 to instruction-blazon, registers, and other fields, at that place are only 12 bits left for immediate values, which will only let for 4096 different values.

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

But there is more. Instead of taking the 12 bits for a single integer, those 12 bits are split into an 8bit number (n) being able to load any viii-flake value in the range of 0-255, and a 4bit rotation field (r) being a right rotate in steps of 2 between 0 and thirty. This means that the full immediate value five is given by the formula: five = n ror two*r. In other words, the only valid immediate values are rotated bytes (values that can exist reduced to a byte rotated past an even number).

Here are some examples of valid and invalid immediate values:

Valid values: #256        // ane ror 24 --> 256 #384        // half-dozen ror 26 --> 384 #484        // 121 ror 30 --> 484 #16384      // 1 ror 18 --> 16384 #2030043136 // 121 ror viii --> 2030043136 #0x06000000 // 6 ror 8 --> 100663296 (0x06000000 in hex)  Invalid values: #370        // 185 ror          31 -->                      31 is not in range (0 – xxx)          #511        // 1 1111 1111          --> chip-pattern can't fit into i byte #0x06010000 // ane 1000 000i.. --> bit-design can't fit into 1 byte

This has the consequence that it is non possible to load a full 32bit address in one go. We can bypass this restrictions by using one of the following 2 options:

1. Construct a larger value out of smaller parts
   1. Instead of using MOV  r0, #511
   2. Split up 511 into ii parts: MOV r0, #256, and ADD r0, #255
2. Use a load construct 'ldr r1,=value' which the assembler volition happily convert into a MOV, or a PC-relative load if that is non possible.
   1. LDR r1, =511

If you attempt to load an invalid immediate value the assembler will complain and output an mistake saying: Fault: invalid constant. If you encounter this mistake, you lot now know what it means and what to practise well-nigh it.
Let's say you want to load #511 into R0.

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

If you try to get together this code, the assembler volition throw an error:

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

You demand to either split 511 in multiple parts or you apply LDR as I described before.

.section .text .global _start  _start:  mov r0, #256   /* 1 ror 24 = 256, and then 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 need to figure out if a certain number tin exist used as a valid firsthand value, y'all don't need to calculate it yourself. You can use my picayune python script called rotator.py which takes your number every bit an input and tells yous if it can be used equally a valid immediate number.

azeria@labs:~$ python rotator.py Enter the value you want to check: 511  Pitiful, 511 cannot be used as an immediate number and has to exist split.  azeria@labs:~$ python rotator.py Enter the value yous desire to cheque: 256  The number 256 can exist used every bit a valid immediate number. 1 ror 24 --> 256

Source: https://azeria-labs.com/memory-instructions-load-and-store-part-4/

Posted by: bellladjecamis.blogspot.com

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