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flat assembler > Non-x86 architectures > [ARM] SysTick on Cortex-M4F (SOLVED, clips+musik)

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hopcode



Joined: 04 Mar 2008
Posts: 563
Location: Germany
[ARM] SysTick on Cortex-M4F (SOLVED, clips+musik)
Hallo,
after solving myself Very Happy successfully an alignment question i come
with the following macros that allow coding such instruction
@mov r0,0xE000ED88

Code:

macro @a4nop {
 if $ and 2
  nop
 end if
}

macro @mov arg,argimm{
 local ..tmp
 @a4nop
 ldr arg,[r15,#0]
 b ..tmp
 dw argimm
 label ..tmp
}


;--- example usage

Code:

.reset_h:
 @mov r0,0xE000ED88
 nop
 nop
 nop
 B .reset_h 


output

Code:
arm disassemble 0x44 7
0x00000044  0x4800      LDR     r0, [pc, #0]    ; 0x00000048
0x00000046  0xe001      B       0x0000004c
0x00000048  0xed88e000  (32-bit Thumb2 ...)
0x0000004c  0xbf00      NOP
0x0000004e  0xbf00      NOP
0x00000050  0xbf00      NOP
0x00000052  0xe7f7      B       0x00000044 


now, i think there is a stall there, for one of two reasons i cannot discriminate:
1) LDR is not listed at page 83 of manual V7M
2) whenever Flash-AHB on I-Code-bus of my STM32F4 can read 64 cache lines of 128 bits, the D-bus has priority on the on it, because fetching literals in the executable section.

EDIT

reason 2) seems consequential but related. the stall explained in chap 3.3.2 Cortex-M4.
LDR Rx,[PC,#imm] might add 1 cycle because of contention with the fetch unit.
add then 2 cycles, because LDR using PC is a blocking operation. the fetch unit should be the one using the D-bus, while I-Bus already busy on a speculative read of the next 2,4 bytes in the code on "normal" memory.

one simpler solution, using MOVW+MOVT,
2 cycles using immediates, and it does not involve fetching from memory.

Code:

0x00000044  0xf64e5088  MOVW r0, #60808  ; 0xed88
0x00000048  0xf2ce0000  MOVT r0, #57344  ; 0xe000


hints ?
Cheers,
Very Happy

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Last edited by hopcode on 18 Jan 2013, 19:09; edited 1 time in total
Post 16 Jan 2013, 17:53
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hopcode



Joined: 04 Mar 2008
Posts: 563
Location: Germany
yea, i was right, without needing to test it, exactly as expected,
here a clip+musik, timing it using the Systick timer
during a debugging session, with fasmarm and openOcd
Very Happy

enjoy,
http://www.youtube.com/watch?v=2Tky6hXkeLE
Cheers,

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Post 18 Jan 2013, 19:08
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