ARM VERSION 1.2 - Manual (Page 70)

Writing ARM and Thumb Assembly Language
2-58 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
2.10.6 Using register-based MAP and FIELD directives
Register-based MAP and FIELD directives define register-based symbols. There are two
main uses for register-based symbols:
• defining structures similar to C structures
• gaining faster access to memory sections described by non register-based MAP and
FIELD directives.
Defining register-based symbols
Register-based symbols can be very useful, but you must be careful when using them.
As a general rule, use them only in the following ways:
• As the location for a load or store instruction to load from or store to. If Location
is a register-based symbol based on the register Rb and with numeric offset, the
assembler automatically translates, for example, LDR Rn,Location into LDR
Rn,[Rb,#offset] .
In an ADR or ADRL instruction, ADR Rn,Location is converted by the assembler into
ADD Rn,Rb,#offset .
• Adding an ordinary numeric expression to a register-based symbol to get another
register-based symbol.
• Subtracting an ordinary numeric expression from a register-based symbol to get
another register-based symbol.
• Subtracting a register-based symbol from another register-based symbol to get an
ordinary numeric expression. Do not do this unless the two register-based
symbols are based on the same register. Otherwise, you have a combination of
two registers and a numeric value. This results in an assembler error.
• As the operand of a :BASE: or :INDEX: operator. These operators are mainly of
use in macros.
Other uses usually result in assembler error messages. For example, if you write LDR
Rn,=Location , where Location is register-based, you are asking the assembler to load Rn
from a memory location that always has the current value of the register Rb plus offset
in it. It cannot do this, because there is no such memory location.
Similarly, if you write ADD Rd,Rn,#expression , and expression is register-based, you are
asking for a single ADD instruction that adds both the base register of the expression and
its offset to Rn . Again, the assembler cannot do this. You must use two ADD instructions
to perform these two additions.