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Hi, Good Day.
I am working on ARM7TDMI core processor from Analog Devices family (ADuC703x). I developed a preemptive scheduler on the same in ARM environment. The functionality of the same is fine and its working well. For the purpose of optimization, i migrated into thumb mode using Keil RV compiler option (by changing the code generation option to THUMB mode and the optimization level option to 3 in C/C++ tab of the target settings option).
After changing the settings my code size is reduced by 2 KB. But, the complete functionality of the software got changed.
Can anybody help me out to get out of this problem? Also, I would like to know why this kind of behavior is occurring... Please let me know your valuable suggesions.
Thanking you in anticipation, Ravi Kumar Desaraju.
Desaraju, Are you trying to increase code density? please not that if you are using a 32 bit data bus, you actually decreased your system performance. Note that not all processor registers are as easily accessable when in thumb mode: R8-R12 are only modifiable via MOV, ADD and CMP. Have you remembered to manually change to thumb mode before your context switch is taking place (can be done by a branch)? Also notice that the multiple-register load-store instructions only support the increment after (IA) addressing mode. Pay attention to the following table, which represents offsets available from a base pointer per instruction when in thumb mode:
LDRB, LDRSB, STRB 0 to 31 bytes LDRH, LDRSH, STRH 0 to 62 bytes LDR, STR 0 to 124 bytes
I am busy with a similar project, never tried it with thumb but I don't see how such a relatively small piece of assembly can win you that much performance when compiled in thumb.
Hi Michel,
Thanks for your response.
Actually, my code is into 2 parts. The scheduling algorithms and task creation part is implemented in C and ISR, Context switching and Wait state machine are developed in assembly. The C code is consumed huge memory in ARM mode (around 3.5KB) and 1.3 KB is assembly (with ARM mode and optimization level as 0).
for the above mentioned figures (ROM), I opted for THUMB mode and observed huge optimization result.
Any ways, the functionality got affected. But, If we use "ARM/THUMB interworking" option from the C/C++ and ASM tab from the target options, I think we can work with any instruction set (ARM/THUMB) since the compiler is taking care about the conversion of instruction from one mode to another mode (ARM to THUMB or THUM to ARM).
Please give me your valuable suggestions if any?
Thanks and Regards, Ravi Kumar Desaraju.
can you explain what you mean by "the functionality got affected"? does you context switch no longer work? checking ARM/Thumb interworking will generate assembly code that can either be executed in ARM or in thumb. It has no effect, as far as I know, on pure assembly code. So if your context switch does not work, you better carefully check whether your assembly code complies with all thumb rules.
Hi,
Yes.... the problem is with context switching only. I am using few 32-bit move instructions in context switching does it affects any functionality in THUMB mode?