ARM PC value after Reset

Question

I am new to MCU and trying to figure out how arm (Cortex M3-M4) based MCU boots. Because booting is specific to any SOC, I took an example hardware board of STM for case study.

Board: STMicroelectronics – STM32L476 32-bit.

In this board when booting mode is (x0)"Boot from User Flash", board maps 0x0000000 address to flash memory address. On flash memory I have pasted my binary with first 4 bytes pointing to vector table first entry, which is esp. Now if I press reset button ARM documentation says PC value will be set to 0x00000000.

CPU generally executes stream of instructions based on PC -> PC + 1 loop. In this case if I see PC value points to esp, which is not instruction. How does Arm CPU does the logic of not use this instruction address, but do a jump to value store at address 0x00000004?

Or this is the case: Reset produces a special hardware interrupt and cause PC value to be value at 0x00000004, if this is the case why Arm documentation says it sets PC value to 0x00000000?

Ref: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka3761.html

What values are in ARM registers after a power-on reset? Applies to: ARM1020/22E, ARM1026EJ-S, ARM1136, ARM720T, ARM7EJ-S, ARM7TDMI, ARM7TDMI-S, ARM920/922T, ARM926EJ-S, ARM940T, ARM946E-S, ARM966E-S, ARM9TDMI

Answer Registers R0 - R14 (including banked registers) and SPSR (in all modes) are undefined after reset.

The Program Counter (PC/R15) will be set to 0x000000, or 0xFFFF0000 if the core has a VINITHI or CFGHIVECS input which is set high as the core leaves reset. This input should be set to reflect where the base of the vector table in your system is located.

The Current Program Status Register (CPSR) will indicate that the ARM core has started in ARM state, Supervisor mode with both FIQ and IRQ mask bits set. The condition code flags will be undefined. Please see the ARM Architecture Manual for a detailed description of the CPSR.

Answer 1

The cortex-m's do not boot the same way the traditional and full sized cores boot. Those at least for the reset as you pointed out fetch from address 0x00000000 (or the alternate if asserted) the first instructions, not really fair to call it the PC value as at this point the PC is somewhat bugus, there are multiple program counters being produced a fake one in r15, one leading the fetching, one doing prefetch, none are really the program counter. anyway, doesnt matter.

The cortex-m as documented in the armv7-m documentation (for the m3 and m4, for the m0 and m0+ see the armv6-m although they so far all boot the same way). These use a vector TABLE not instructions. The CORE reads address 0x00000000 (or an alternate if a strap is asserted) and that 32 bit value gets loaded into the stack pointer register. it reads address 0x00000004 it checks the lsbit (maybe not all cores do) if set then this is a valid thumb address, strips the lsbit off (makes it a zero) and begins to fetch the first instructions for the reset handler at that address so if your flash starts with

0x00000000 : 0x20001000
0x00000004 : 0x00000101

the cortex-m will put 0x20001000 in the stack pointer and fetch the first instructions from address 0x100. Being thumb instructions are 16 bits with thumb2 extensions being two 16 bit portions, its not an x86 the program counter is aligned for the full sized processors with 32 bit instructions it fetches on aligned addresses 0x0000, 0x0004, 0x0008 it doesnt increment pc <= pc + 1; For thumb mode or thumb processors it is pc = pc + 2. But also the fetches are not necessarily single instruction transactions, for the full sized they may fetch 4 or 8 words per transaction, the cortex-ms as documented in the technical reference manuals some are able to be compiled or strapped to 16 bits at a time or 32 bits at a time. So no need to talk about or think about execution loops fetching pc = pc + 1, that doesnt make sense even in an x86 these days.

to be fair arms documentation is generally good, on the better side compared to a number of others, not the best. Unlike the full sized arm exception table, the vector table in the cortex-m documentation was not done as well as it could have been, could have/should have just done something like the full sized but shown they were vectors not instructions. It is in there though in the architectural reference manual for the armv6-m and armv7-m (and I would assume armv8-m as well but have not looked, got some parts last week but boards are not here yet, will know very soon). Cant look for words like reset have to look for interrupt or undefined or hardfault, etc in that manual.

EDIT

unwrap your mind on this notion of how the processor starts fetching, it can be any arbitrary address they add into the design, and then the execution of the instructions determines the next address and next address, etc.

Also understand unlike say x86 or microchip pic or the avrs, etc, the core and the chips are two different companies. Even in those same company designs, but certainly where there is a clear division between the IP with a known bus, the ARM CORE will read address 0x00000004 on the AMBA/AXI/AHB bus, the chip vendor can mirror that address in as many different places as they want, in this case with the stm32 there probably isnt actually anything at 0x00000000 as their documentation implies based on the boot pins they map it either to an internal bootloader, or they map it to the user application at 0x08000000 (or in most stm32's if there is an exception thats fine I have not yet seen it) so when strapped that way and the logic has those addresses mirrored you will see the same 32 bit values at 0x00000000 and 0x08000000, 0x00000004 and 0x08000004 and so on for some limited amount of address space. This is why even though linking for 0x00000000 will work to some extent (till you hit that limit which is probably smaller than the application flash size), you will see most folks link for 0x08000000 and the hardware takes care of the rest, so your table really wants to look like

0x08000000 : 0x20001000
0x08000004 : 0x08000101

for an stm32, at least the dozens I have seen so far.

The processor reads 0x00000000 which is mirrored to the first item in the application flash, finds 0x20001000, it then reads 0x00000004 which is mirroed to the second word in the application flash and gets 0x08000101 which causes a fetch from 0x08000100 and now we are executing from the proper fully mapped application flash address space. so long as you dont change the mirroring, which I dont know if you can on an stm32 (nxp chips you can and I dont know about ti or other brands off hand). Some of the cortex-m cores the VTOR register is there and changable (others it is fixed at 0x00000000 and you cant change it), you do not need to change it to 0x08000000 for an stm32, at least all the ones I know about. its only if you are actively changing the mirroring of the zero address space yourself if possible or if you say have your own bootloader and maybe YOUR application space is 0x08004000 and that application wants a vector table of its own. then you either use VTOR or you build the bootloaders vector table such that it runs code that reads the vectors at 0x08004000 and branches to those. The NXP and others in the past certainly with the ARMV7TDMI cores, would let you change the mirroring of address zero because those older cores didnt have a programmable vector table offset register, helping you solve that problem in their chip designs. Newer ARM cores with a VTOR eliminate that need and over time the chip vendors might not bother anymore if they do at all...

EDIT

I dont know if you have the discovery board or the nucleo, I assume the latter as the former is not available (wish I knew about that one would like to have one. And/or I already have one and its buried in a drawer and I never got to it).

so here is a somewhat minimal program you can try on your stm32

.cpu cortex-m0
.thumb
.globl _start
_start:
.word 0x20000400
.word reset
.word loop
.word loop
.thumb_func
loop: b loop
.thumb_func
reset:
    ldr r0,=0x20000000
    mov r2,sp
    str r2,[r0]
    add r0,r0,#4
    mov r2,pc
    str r2,[r0]
    add r0,r0,#4
    mov r1,#0
top:
    str r1,[r0]
    add r1,r1,#1
    b top

build

arm-none-eabi-as so.s -o so.o
arm-none-eabi-ld -Ttext=0x08000000 so.o -o so.elf
arm-none-eabi-objdump -D so.elf > so.list
arm-none-eabi-objcopy so.elf -O binary so.bin

this should build with arm-linux-whatever- or other arm-whatever-whatever tools from a binutils from the last 10 years.

The disassembly is important to examine before using the binary, dont want to brick your chip (with an stm32 there is a way to get unbricked)

08000000 <_start>:
 8000000:   20000400    andcs   r0, r0, r0, lsl #8
 8000004:   08000013    stmdaeq r0, {r0, r1, r4}
 8000008:   08000011    stmdaeq r0, {r0, r4}
 800000c:   08000011    stmdaeq r0, {r0, r4}

08000010 <loop>:
 8000010:   e7fe        b.n 8000010 <loop>

08000012 <reset>:
 8000012:   4805        ldr r0, [pc, #20]   ; (8000028 <top+0x6>)
 8000014:   466a        mov r2, sp
 8000016:   6002        str r2, [r0, #0]
 8000018:   3004        adds    r0, #4
 800001a:   467a        mov r2, pc
 800001c:   6002        str r2, [r0, #0]
 800001e:   3004        adds    r0, #4
 8000020:   2100        movs    r1, #0

08000022 <top>:
 8000022:   6001        str r1, [r0, #0]
 8000024:   3101        adds    r1, #1
 8000026:   e7fc        b.n 8000022 <top>
 8000028:   20000000    andcs   r0, r0, r0

the disassembler doesnt know that the vector table is not instructions so you can ignore those.

08000000 <_start>:
 8000000:   20000400
 8000004:   08000013
 8000008:   08000011
 800000c:   08000011

08000010 <loop>:
 8000010:   e7fe        b.n 8000010 <loop>

08000012 <reset>:

Does it start the vector table at 0x08000000, check. Our stack pointer init value is at 0x00000000, yes, the reset vector we had the tools place for us. thumb_func tells them the following label is an address for some code/function/procedure/whatever_not_data so they orr the one on there for us. our reset handler is at address 0x08000012 so we want to see 0x08000013 in the vector table, check. I tossed in a couple more for demonstration purposes, sent them to an infinite loop at address 0x08000010 so the vector table should have 0x08000011, check.

So assuming you have a nucleo board not the discovery then you can copy the so.bin file to the thumb drive that shows up when you plug it in.

If you use openocd to connect through the stlink interface into the board now you can see that it was running (details left to the reader to figure out)

Open On-Chip Debugger
> halt
stm32f0x.cpu: target state: halted
target halted due to debug-request, current mode: Thread 
xPSR: 0x01000000 pc: 0x08000022 msp: 0x20000400
> mdw 0x20000000 20
0x20000000: 20000400 0800001e 0048cd01 200002e7 200002e9 200002eb 200002ed 00000000 
0x20000020: 00000000 00000000 00000000 200002f1 200002ef 00000000 200002f3 200002f5 
0x20000040: 200002f7 200002f9 200002fb 200002fd 
> resume
> halt
stm32f0x.cpu: target state: halted
target halted due to debug-request, current mode: Thread 
xPSR: 0x01000000 pc: 0x08000022 msp: 0x20000400
> mdw 0x20000000 20
0x20000000: 20000400 0800001e 005e168c 200002e7 200002e9 200002eb 200002ed 00000000 
0x20000020: 00000000 00000000 00000000 200002f1 200002ef 00000000 200002f3 200002f5 
0x20000040: 200002f7 200002f9 200002fb 200002fd 

so we can see that the stack pointer had 0x20000400 as expected

0x20000000: 20000400 0800001e 0048cd01

the program counter which is not some magical thing, they have to somewhat fake it to make the instruction set work.

 800001a:   467a        mov r2, pc

as defined in the instruction set the pc value used in this instruction is two instructions ahead of the address of this instruction, so 0x0800001A + 4 = 0x0800001E which is what we see in the memory dump.

And the third item is a counter showing we are running, the resume and halt shows that that count kept going

0x20000000: 20000400 0800001e 005e168

So this demonstrates, the vector table, initializing the stack pointer, the reset vector, where code execution starts, what the value of the pc is at some point in the program, and seeing the program run.

the .cpu cortex-m0 makes it build the most compatible program for the cortex-m family and the mov r0,=0x20000000 was cheating, you posted the same feature in your comment it says I want to load the address of blah into the register a label is just an address and they let you put just an address =_estack is the address of a label =0x20000000 is just a number treated as an address (addresses are just numbers as well, nothing magical about them). I could have done a smaller immediate with a shift or explicitly have done the pc relative load. force of habit in this case.

EDIT2

In attempt for a programmer to understand that the chip is logic, only some percentage of it is software/instruction driven, even within that it is just logic that does more things than the software instruction itself indicates. You want to read from memory your instruction asks the processor to do it but in a real chip there are a number of steps involved to actually perform that, microcoded or not (ARMs are not microcoded) there are state machines that walk through the various steps to perform each of these tasks. grab the values from registers, compute the address, do the memory transaction which is a handful of separate steps, take the return value and place it in the register file.

.thumb
.globl _start
_start:
.word 0x20001000
.word reset
.word loop
.word loop
.thumb_func
loop: b loop
.thumb_func
reset:
    ldr r0,loop_counts
loop_top:
    sub r0,r0,#1
    bne loop_top
    b reset
.align
loop_counts: .word 0x1234


00000000 <_start>:
   0:   20001000    andcs   r1, r0, r0
   4:   00000013    andeq   r0, r0, r3, lsl r0
   8:   00000011    andeq   r0, r0, r1, lsl r0
   c:   00000011    andeq   r0, r0, r1, lsl r0

00000010 <loop>:
  10:   e7fe        b.n 10 <loop>

00000012 <reset>:
  12:   4802        ldr r0, [pc, #8]    ; (1c <loop_counts>)

00000014 <loop_top>:
  14:   3801        subs    r0, #1
  16:   d1fd        bne.n   14 <loop_top>
  18:   e7fb        b.n 12 <reset>
  1a:   46c0        nop         ; (mov r8, r8)

0000001c <loop_counts>:
  1c:   00001234    andeq   r1, r0, r4, lsr r2

Just barely enough of an instruction set simulator to run that program.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


#define ROMMASK 0xFFFF
#define RAMMASK 0xFFF

unsigned short rom[ROMMASK+1];
unsigned short ram[RAMMASK+1];

unsigned int reg[16];
unsigned int pc;
unsigned int cpsr;
unsigned int inst;

int main ( void )
{
    unsigned int ra;
    unsigned int rb;
    unsigned int rc;
    unsigned int rx;

    //just putting something there, a real chip might have an MBIST, might not.
    memset(reg,0xBA,sizeof(reg));
    memset(ram,0xCA,sizeof(ram));
    memset(rom,0xFF,sizeof(rom));

    //in a real chip the rom/flash would contain the program and not
    //need to do anything to it, this sim needs to have the program
    //various ways to have done this...

                            //00000000 <_start>:
    rom[0x00>>1]=0x1000;    //   0: 20001000    andcs   r1, r0, r0
    rom[0x02>>1]=0x2000;
    rom[0x04>>1]=0x0013;    //   4: 00000013    andeq   r0, r0, r3, lsl r0
    rom[0x06>>1]=0x0000;
    rom[0x08>>1]=0x0011;    //   8: 00000011    andeq   r0, r0, r1, lsl r0
    rom[0x0A>>1]=0x0000;
    rom[0x0C>>1]=0x0011;    //   c: 00000011    andeq   r0, r0, r1, lsl r0
    rom[0x0E>>1]=0x0000;
                            //
                            //00000010 <loop>:
    rom[0x10>>1]=0xe7fe;    //  10: e7fe        b.n 10 <loop>
                            //
                            //00000012 <reset>:
    rom[0x12>>1]=0x4802;    //  12: 4802        ldr r0, [pc, #8]    ; (1c <loop_counts>)
                            //
                            //00000014 <loop_top>:
    rom[0x14>>1]=0x3801;     //  14:    3801        subs    r0, #1
    rom[0x16>>1]=0xd1fd;     //  16:    d1fd        bne.n   14 <loop_top>
    rom[0x18>>1]=0xe7fb;     //  18:    e7fb        b.n 12 <reset>
    rom[0x1A>>1]=0x46c0;     //  1a:    46c0        nop         ; (mov r8, r8)
                            //
                            //0000001c <loop_counts>:
    rom[0x1C>>1]=0x0004;     //  1c:    00001234    andeq   r1, r0, r4, lsr r2
    rom[0x1E>>1]=0x0000;


    //reset
    //THIS IS NOT SOFTWARE DRIVEN LOGIC, IT IS JUST LOGIC
    ra=rom[0x00>>1];
    rb=rom[0x02>>1];
    reg[14]=(rb<<16)|ra;
    ra=rom[0x04>>1];
    rb=rom[0x06>>1];
    rc=(rb<<16)|ra;
    if((rc&1)==0) return(1); //normally run a fault handler here
    pc=rc&0xFFFFFFFE;
    reg[15]=pc+2;
    cpsr=0x000000E0;

    //run
    //THIS PART BELOW IS SOFTWARE DRIVEN LOGIC
    //still you can see that each instruction requires some amount of
    //non-software driven logic.
    //while(1)
    for(rx=0;rx<20;rx++)
    {
        inst=rom[(pc>>1)&ROMMASK];
printf("0x%08X : 0x%04X\n",pc,inst);        
        reg[15]=pc+4;
        pc+=2;
        if((inst&0xF800)==0x4800)
        {
            //LDR
printf("LDR r%02u,[PC+0x%08X]",(inst>>8)&0x7,(inst&0xFF)<<2);
            ra=(inst>>0)&0xFF;
            rb=reg[15]&0xFFFFFFFC;
            ra=rb+(ra<<2);
printf(" {0x%08X}",ra);            
            rb=rom[((ra>>1)+0)&ROMMASK];
            rc=rom[((ra>>1)+1)&ROMMASK];
            ra=(inst>>8)&0x07;
            reg[ra]=(rc<<16)|rb;
printf(" {0x%08X}\n",reg[ra]);            
            continue;
        }
        if((inst&0xF800)==0x3800)
        {
            //SUB
            ra=(inst>>8)&0x07;
            rb=(inst>>0)&0xFF;
printf("SUBS r%u,%u ",ra,rb);
            rc=reg[ra];
            rc-=rb;
            reg[ra]=rc;
printf("{0x%08X}\n",rc);            
            //do flags
            if(rc==0) cpsr|=0x80000000; else cpsr&=(~0x80000000); //N flag
            //dont need other flags for this example
            continue;
        }
        if((inst&0xF000)==0xD000) //B conditional
        {
            if(((inst>>8)&0xF)==0x1) //NE
            {
                ra=(inst>>0)&0xFF;
                if(ra&0x80) ra|=0xFFFFFF00;
                rb=reg[15]+(ra<<1);
printf("BNE 0x%08X\n",rb);
                if((cpsr&0x80000000)==0)
                {
                    pc=rb;
                }
                continue;
            }
        }
        if((inst&0xF000)==0xE000) //B 
        {
            ra=(inst>>0)&0x7FF;
            if(ra&0x400) ra|=0xFFFFF800;
            rb=reg[15]+(ra<<1);
printf("B 0x%08X\n",rb);
            pc=rb;
            continue;
        }

        printf("UNDEFINED INSTRUCTION 0x%08X: 0x%04X\n",pc-2,inst);
        break;
    }
    return(0);
}

You are welcome to hate my coding style, this is a brute force thrown together for this question thing. No I dont work for ARM, this can all be pulled from public documents/information. I shortened the loop to 4 counts to see it hit the outer loop

0x00000012 : 0x4802
LDR r00,[PC+0x00000008] {0x0000001C} {0x00000004}
0x00000014 : 0x3801
SUBS r0,1 {0x00000003}
0x00000016 : 0xD1FD
BNE 0x00000014
0x00000014 : 0x3801
SUBS r0,1 {0x00000002}
0x00000016 : 0xD1FD
BNE 0x00000014
0x00000014 : 0x3801
SUBS r0,1 {0x00000001}
0x00000016 : 0xD1FD
BNE 0x00000014
0x00000014 : 0x3801
SUBS r0,1 {0x00000000}
0x00000016 : 0xD1FD
BNE 0x00000014
0x00000018 : 0xE7FB
B 0x00000012
0x00000012 : 0x4802
LDR r00,[PC+0x00000008] {0x0000001C} {0x00000004}
0x00000014 : 0x3801
SUBS r0,1 {0x00000003}
0x00000016 : 0xD1FD
BNE 0x00000014
0x00000014 : 0x3801
SUBS r0,1 {0x00000002}
0x00000016 : 0xD1FD
BNE 0x00000014
0x00000014 : 0x3801
SUBS r0,1 {0x00000001}
0x00000016 : 0xD1FD
BNE 0x00000014
0x00000014 : 0x3801
SUBS r0,1 {0x00000000}
0x00000016 : 0xD1FD
BNE 0x00000014
0x00000018 : 0xE7FB
B 0x00000012

Perhaps this helps perhaps this makes it worse. Most of the logic is not driven by instructions, each instruction, requires some amount of logic not counting the common logic like instruction fetching and things like that.

If you add more code this simulator will break it ONLY supports these handful of instructions and this loop.