ARM VERSION 1.2 Using register-based MAP and Field directives, Defining register-based symbols

Writing ARM and Thumb Assembly Language

2.10.6Using 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.

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