The difference between B jump instruction and LDR jump in ARM assembly

• The B jump instruction is independent of the code position. After assembly, it will be replaced by the current PC value plus (minus) a correction value. No matter which address this instruction is executed at, it can jump to the specified location.
• B can only jump within the 32M range of the current PC, and LDR can only jump within the 4KB (0xfff range) of the current PC.
• The LDR PC, =xxx instruction will directly load the value of a label xxx into PC, but the label will be replaced with a value corresponding to RO after compilation, so that it can jump to a specified location no matter where the instruction is executed.
• Take the startup code snippet of AT91SAM9260 as an example, 0x10000000 is the Flash base address, and 0x20000000 is the SDRAM base address:

         ENTRY is the starting point, which means the offset of this code is 0. Let HandlerReset's offset be offset. If this program is compiled according to RO=0x10000000:

         b HandlerReset <=> ADD PC, PC, #offset

         LDR PC, =HandlerReset <=> MOV PC, #(RO+offset)

When the system is reset, b HandlerReset points PC to the location of the Flash image code at address 0; while LDR PC, =HandlerReset points PC to the original code location in Flash, so both can execute normally.

        However, if the program is compiled according to RO=0x20000000, the code generated after compilation still needs to be burned into Flash, that is, address 0x10000000. After the system is reset, it will be executed from address 0. b HandlerReset still executes the Flash image code, and the program can be executed normally. However, LDR PC, = HandlerReset will make PC point to 0x20000000+offset, which is located in SDRAM. At this time, the code has not been copied, and there is no code in SDRAM, so the program cannot run.