ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Assembler syntax
LDRSH , [ {, #+/-}] Offset: index==TRUE, wback==FALSE
LDRSH , [, #+/-]! Pre-indexed: index==TRUE, wback==TRUE
LDRSH , [], #+/- Post-indexed: index==FALSE, wback==TRUE
where:
See Standard assembler syntax fields on page A8-7.
The destination register.
Instruction Details
The base register. The SP can be used. For PC use see LDRSH (literal) on page A8-170.
+/- Is + or omitted to indicate that the immediate offset is added to the base register value
(add == TRUE), or – to indicate that the offset is to be subtracted (add == FALSE). Different
instructions are generated for #0 and #-0.
The immediate offset used to form the address, Values are 0-4095 for encoding T1, and
0-255 for encoding T2 or A1. For the offset syntax, can be omitted, meaning an offset
of 0.
The pre-UAL syntax LDRSH is equivalent to LDRSH.
Operation
if ConditionPassed() then
EncodingSpecificOperations(); NullCheckIfThumbEE(n);
offset_addr = if add then (R[n] + imm32) else (R[n] - imm32);
address = if index then offset_addr else R[n];
data = MemU[address,2];
if wback then R[n] = offset_addr;
if UnalignedSupport() || address = ‘0’ then
R[t] = SignExtend(data, 32);
else // Can only apply before ARMv7
R[t] = bits(32) UNKNOWN;
Exceptions
Data Abort.
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. A8-169