Unaligned address access in ARMv7

B3.7 Memory region attributes.

        MRS     r0, CPSR
        ORR     r0, r0, #(PSR_I_BIT|PSR_F_BIT)  ; Disable IRQ & FIQ
        MSR     CPSR_c, r0
; Disable MMU stuff and caches
        MRC     p15, 0, r0, c1, c0, 0           ; read control register to r0
        BIC     r0, r0, #0x000f                 ; clear W(Write buffer), C(Cache), A(Align), M(MMU) bits
        BIC     r0, r0, #0x1100                 ; clear I(ICache), S(System protection) bits
        MCR     p15, 0, r0, c1, c0, 0           ; write control register.
                
stack_setup
        LDR     r0, =|Image$$ZI$$Limit|
        ADD     r0, r0, #STACK_SIZE_SVC
                
        MSR     CPSR_c, #(MODE_SVC :OR: PSR_F_BIT :OR: PSR_I_BIT)
        MOV     sp, r0
mmu_setup        
        LDR     r2, =MMU_TABLE_BASE                             ; mmu page base address
        MCR     p15, 0, r2, c2, c0, 0           ; write mmu page base address
                
        BL      _cpu_mmu_init
        
                
        LDR     r0, =0x89
        LDRH r1, [r0]

In Realview, if the target processor is ARMv6, ARMv7-A (such as Cortex-A8), or ARMv7-R, the default option of the compiler is --unaligned_access. If a version before ARMv6 or a processor of the ARMv7-M system is selected, the default option is --no_unaligned_access.

For the option --no_unaligned_access, the compiler will automatically convert the C language access to variables with unaligned addresses into several byte operations. For example:

__packed struct usb_endpoint_descriptor {
    u8 bLength;
    u16 wMaxPacketSize;
    u8 bDescriptorType;
    u8 bEndpointAddress;
    u8 bmAttributes;
    u8 bInterval;
} ;
int main(void)
{   
    volatile struct usb_endpoint_descriptor ep;
   
    ep.wMaxPacketSize = 0x200;
    while(1);
    return 0;
}
If compiled with the --no_unaligned_access option, its disassembled code is as follows:
    main
        0xe100c5f0:    e92d400c    .@-.    PUSH     {r2,r3,lr}
        0xe100c5f4:    e3a00c02    ....    MOV      r0,#0x200
        0xe100c5f8:    e5cd0001    ....    STRB     r0,[sp,#1]
        0xe100c5fc:    e1a00420     ...    LSR      r0,r0,#8
        0xe100c600:    e5cd0002    ....    STRB     r0,[sp,#2] 
        0xe100c604:    eafffffe    ....    B        0xe100c604 ; main + 20
If compiled with the --unaligned_access option, its disassembled code is as follows:
    main
        0xe100c5f0:    e92d400c    .@-.    PUSH     {r2,r3,lr}
        0xe100c5f4:    e3a00c02    ....    MOV      r0,#0x200
        0xe100c5f8:    e1cd00b1    ....    STRH     r0,[sp,#1] 
        0xe100c5fc:    eafffffe    ....    B        0xe100c5fc ; main + 12

If your target platform is ARMv6 or ARMv7-AR, but the MMU is disabled in your code, you should manually add the compilation option --no_unaligned_access to let the compiler handle unaligned access for you.