AM6548: SBBO Instrution of PRU

Thomas Dorsch

Part Number: AM6548

Hi,

I have to debug a PRU program which runs on AM6548. To understand exactly what happens I try to follow the assembler code. But at a certain SBBO instruction, I can't see the change/new value in memory.

C Code Line 94. Instruction 0x3d0 got executed at last step.
I think the PRU should copy 4 Bytes starting from register R1.b0 to address which is stored in R14 plus offset of 8. Is this correct?

If yes, I should see at memory address 0xFFFFFFAA + 0x8 the value 0xFFFFFFFF. I am wondering about the high address, according my linker script only PRU_DMEM_0_1 is used, which starts at address 0x0 and is 4kB big.

I'm sure I miss a simple mistake...

Best Regards,
Thomas

over 6 years ago

0 Ruediger Wurth over 6 years ago

Intellectual 705 points

Hello Thomas,
did you miss the .b0 in the LBBO &R14.b0 ?
I guess with the .b0 only the lowest byte of R14 is taken, resulting in address 0x000000AA
Regards,
Ruediger

0 Thomas Dorsch over 6 years ago in reply to Ruediger Wurth

Expert 2601 points

Hi Rüdiger,

thanks for you quick response. I am not 100% sure with your guess. Maybe an TI member can confirm this.

As I found out later , I messed up the stack size. It was to small for the program. Increasing the stack size fixed the issue with the weird addresses.

0 Garrett Ding over 6 years ago in reply to Thomas Dorsch

TI__Mastermind 43296 points

Hi Thomas,

Good to know you have resolved the issue. With stack overflow, some weird behavior may happen.

For LBBO instruction, see processors.wiki.ti.com/.../PRU_Assembly_Instructions

LBBO &REG1, Rn2, OP(255), IM(124)
operation:
memcpy( offset(REG1), Rn2+OP(255), IM(124) );

so LBBO.R14.b0, R2, 0, 4 actually copy 4 bytes from memory address R2 to destination register R14 (from .b0 offset).

This example 'SBBO R0.b2, R2, 0, 5' may help understand the 'offset(REG1)': processors.wiki.ti.com/.../PRU_Assembly_Advanced_Topics

Regards,
Garrett

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AM6548: SBBO Instrution of PRU