The assembler uses literal pools to hold certain constant
values that are to be loaded into registers. The assembler places
a literal pool at the end of each section. The end of a section
is defined either by the
END directive at the end of
the assembly or by the
AREA directive at the start of
the following section. The
END directive at the end
of an included file does not signal the end of a section.
In large sections the default literal pool can be out of range
of one or more
LDR instructions. The offset from the
PC to the constant must be:
less than 4KB in ARM or Thumb code when the 32-bit
LDRinstruction is available, but can be in either direction
forward and less than 1KB when only the 16-bit Thumb
LDRinstruction is available.
LDR Rd,=const pseudo-instruction requires
the immediate value to be placed in a literal pool, the assembler:
Checks if the value is available and addressable in any previous literal pools. If so, it addresses the existing constant.
Attempts to place the value in the next literal pool if it is not already available.
If the next literal pool is out of range, the assembler generates
an error message. In this case you must use the
to place an additional literal pool in the code. Place the
after the failed
LDR pseudo-instruction, and within
the valid range for an
You must place literal pools where the processor does not attempt to execute them as instructions. Place them after unconditional branch instructions, or after the return instruction at the end of a subroutine. Example 9 shows how this works.
The instructions listed as comments are the ARM instructions generated by the assembler.
AREA Loadcon, CODE, READONLY ENTRY ; Mark first instruction to execute start BL func1 ; Branch to first subroutine BL func2 ; Branch to second subroutine stop MOV r0, #0x18 ; angel_SWIreason_ReportException LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit SVC #0x123456 ; ARM semihosting (formerly SWI) func1 LDR r0, =42 ; => MOV R0, #42 LDR r1, =0x55555555 ; => LDR R1, [PC, #offset to ; Literal Pool 1] LDR r2, =0xFFFFFFFF ; => MVN R2, #0 BX lr LTORG ; Literal Pool 1 contains ; literal Ox55555555 func2 LDR r3, =0x55555555 ; => LDR R3, [PC, #offset to ; Literal Pool 1] ; LDR r4, =0x66666666 ; If this is uncommented it ; fails, because Literal Pool 2 ; is out of reach BX lr LargeTable SPACE 4200 ; Starting at the current location, ; clears a 4200 byte area of memory ; to zero END ; Literal Pool 2 is empty