Databases
StackOverflow2013
Postsdbo
POARM926ej-s reset handler - does it overwrite itself?
Body
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POARM926ej-s reset handler - does it overwrite itself?
    text
    copied!<p>I know that ARM926ej-s is using <strong>full descending</strong> stack. Now I have this startup code in which i have comented using <code>/**/</code> on lines that are relevant for this question.</p> <pre><code>.global __start .global arm926ejs_reset .global dcache_flush .global __gnu_bssstart .global __gnu_bssend .global __image_size .EQU MODE_USR, 0x010 .EQU MODE_FIQ, 0x011 .EQU MODE_IRQ, 0x012 .EQU MODE_SVC, 0x013 .EQU MODE_SVC_NI, 0x0D3 .EQU MODE_ABORT, 0x017 .EQU MODE_UNDEF, 0x01b .EQU MODE_SYSTEM, 0x01f .EQU MODE_BITS, 0x01f .EQU I_MASK, 0x080 .EQU F_MASK, 0x040 .EQU IF_MASK, 0x0C0 .EQU BROM_MMU_BASE_ADDR, 0x1201C000 .EQU MMU_DISABLE_MASK, 0xFFFFEFFA .EQU MMU_ENABLE_MASK, 0x00001005 .EQU FIQ_STACK_SIZE, 128 .EQU IRQ_STACK_SIZE, 256 .EQU ABORT_STACK_SIZE, 32 .EQU UNDEF_STACK_SIZE, 32 .EQU SYSTEM_STACK_SIZE, 64 .text .code 32 .align 2 __start: arm926ejs_reset: B arm926ejs_reset_handler /*reset vector*/ .word 0x41676d69 .word 0,0,0,0,0 image_type: .word 0x0000000A sizeOfPermanentCode: .word (__image_size) .word 0,0 bootparameter: .word 0 .word 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 arm926ejs_reset_handler: MOV r0, #MODE_SVC_NI MSR cpsr_cxsf, r0 MRC p15, 0, r1, c1, c0, 0 LDR r2,=MMU_DISABLE_MASK AND r1, r1, r2 MCR p15, 0, r1, c1, c0, 0 MOV r1,#0 MCR p15, 0, r1, c8, c7, 0 MCR p15, 0, r1, c7, c7, 0 MRC p15, 0, r1, c1, c0, 0 LDR r2,=0x1000 ORR r1, r1, r2 MCR p15, 0, r1, c1, c0, 0 MVN r1, #0 MCR p15, 0, r1, c3, c0, 0 LDR r3, =BROM_MMU_BASE_ADDR MCR p15, 0, r3, c2, c0, 0 LDR r5, =inVirtMem MRC p15, 0, r1, c1, c0, 0 LDR r2,=MMU_ENABLE_MASK ORR r1, r1, r2 MCR p15, 0, r1, c1, c0, 0 MOV pc, r5 NOP NOP NOP inVirtMem: ADR r3, arm926ejs_reset /*we store the reset vector address*/ SUB r3, r3, #4 /*why do we then substract 4 bytes?*/ MOV r1, #IF_MASK ORR r0, r1, #MODE_FIQ /*sets stack for MCPU mode*/ MSR cpsr_cxsf, r0 MOV sp, r3 SUB r3, r3, #FIQ_STACK_SIZE ORR r0, r1, #MODE_IRQ /*sets stack for MCPU mode*/ MSR cpsr_cxsf, r0 MOV sp, r3 SUB r3, r3, #IRQ_STACK_SIZE ORR r0, r1, #MODE_ABORT /*sets stack for MCPU mode*/ MSR cpsr_cxsf, r0 MOV sp, r3 SUB r3, r3, #ABORT_STACK_SIZE ORR r0, r1, #MODE_UNDEF /*sets stack for MCPU mode*/ MSR cpsr_cxsf, r0 MOV sp, r3 SUB r3, r3, #UNDEF_STACK_SIZE ORR r0, r1, #MODE_SYSTEM /*sets stack for MCPU mode*/ MSR cpsr_cxsf, r0 MOV sp, r3 SUB r3, r3, #SYSTEM_STACK_SIZE ORR r0, r1, #MODE_SVC /*sets stack for MCPU mode*/ MSR cpsr_cxsf, r0 MOV sp, r3 LDR r0, =__gnu_bssstart LDR r1, =__gnu_bssend MOV r2, #0 clearzi: CMP r0, r1 BEQ clearzi_exit STR r2, [r0] ADD r0, r0, #4 B clearzi clearzi_exit: load_entry: LDR pc, =c_entry NOP dcache_flush: CMP r0, #0 BNE inval flushonly: MRC p15, 0, r15, c7, c10, 3 BNE flushonly MOV pc, lr inval: MRC p15, 0, r15, c7, c14, 3 BNE inval MOV pc, lr .END </code></pre> <p>In the code above the first comment shows a reset vector (a branching instruction <code>B arm926ejs_reset_handler</code>) immediately followed by a header and some commands to prepare the mcu...</p> <p>Now take a look at the next comment where we store the reset vector's address into register <code>r3</code> and in the next line we substract the <code>4</code> bytes from this address. </p> <p><strong>Q1:</strong> Why do we do this? Does this address now show at the location of the <code>.word 0x41676d69</code> ?</p> <p>In next lines there are multiple simmilar blocks and each sets the stack for different mcpu modes. Lets take a look at the first one where stack pointer <code>sp</code> is loaded with register <code>r3</code> and then the size of the stack is substracted from this address to get the stack size for this mcpu mode.</p> <p><strong>Q2:</strong> Does this stack overwrite the header from <code>.word 0x41676d69</code> on and all the instructions used to prepare the mcu? Does reset vector <code>B arm926ejs_reset_handler</code> stay intact? Why do we owerwrite header? Don't we need it anymore?</p>
 

Querying!

 
Guidance
An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload