Greetings:
I would like to learn a macro technique for the
address assignment of single bit variables to the
bitBand region of an ARM Cortex M3 MPU.
I wrote the following macro for defining and accessing
single bit semaphore variables:
#define SRBB(varName, bit) (*((volatile unsigned int *)\
((((uint32_t)&varName - SRAM_BASE)*32)+(bit*4)+SRAM_BB_BASE)))
where:
&varName = 0x20000298 // for this example
bit = 1 // for this example
SRAM_BASE = 0x20000000
SRAM_BB_BASE = 0x22000000
For a reference I pre-calculated the bitBand address
and took a look at the associated ASM steps. As shown in
example 1 the GCC compiler matched the textbook
efficiency of 3 ASM instructions for setting the bit.
....
1) Example with manually calculated bitBand address:
(*((volatile unsigned int *)0x22005304)) = (uint16_t)1;;
4A20 ldr r2, [pc, #0x080]
F04F0301 mov.w r3, #0x00000001
6013 str r3, [r2, #0x00]
....
I then decided to generate the bitBand address with my
macro and discovered that it took 7 instruction cycles to
set the bit as some of the address arithmetic was assigned
by GCC to the STM32 processor.
....
2)Example that uses the macro:
HEARTBEAT_500MS_BBFLG = 1;
4B22 ldr r3, [pc, #0x088]
EA4F1343 mov.w r3, r3, lsl #0x05
F1035308 add.w r3, r3, #0x22000000
F1030304 add.w r3, r3, #0x00000004
461A mov r2, r3
F04F0301 mov.w r3, #0x00000001
6013 str r3, [r2, #0x00]
....
It appears the compiler pre calculates the 0x298 byte
offset and bit offsets. Then the 32 times multiplication
and offset sum steps are passed to the STM32.
I wonder if someone here knows why the GCC
pre-processor does not complete my macro calculation?
If so please advise me or point me to information that
describes what I need to change so that the GCC
pre-processor will complete the macro calculation
without adding STM32 instructions.
Thanks in advance for any help!
Best Regards,
Tom Alldread
