15.29 FIELD

The FIELD directive describes space within a storage map that has been defined using the MAP directive.

Syntax

{label} FIELD expr

where:

label

is an optional label. If specified, label is assigned the value of the storage location counter, {VAR}. The storage location counter is then incremented by the value of expr.

expr

is an expression that evaluates to the number of bytes to increment the storage counter.

Usage

If a storage map is set by a MAP directive that specifies a base-register, the base register is implicit in all labels defined by following FIELD directives, until the next MAP directive. These register-relative labels can be quoted in load and store instructions.

# is a synonym for FIELD.

Examples

The following example shows how register-relative labels are defined using the MAP and FIELD directives:

    MAP     0,r9        ; set {VAR} to the address stored in R9
    FIELD   4           ; increment {VAR} by 4 bytes
Lab FIELD   4           ; set Lab to the address [R9 + 4] 
                        ; and then increment {VAR} by 4 bytes
    LDR     r0,Lab      ; equivalent to LDR r0,[r9,#4]

When using the MAP and FIELD directives, you must ensure that the values are consistent in both passes. The following example shows a use of MAP and FIELD that causes inconsistent values for the symbol x. In the first pass sym is not defined, so x is at 0x04+R9. In the second pass, sym is defined, so x is at 0x00+R0. This example results in an assembly error.

    MAP 0, r0
    if :LNOT: :DEF: sym
      MAP 0, r9
      FIELD 4  ; x is at 0x04+R9 in first pass
    ENDIF
x   FIELD 4    ; x is at 0x00+R0 in second pass
sym LDR r0, x  ; inconsistent values for x results in assembly error
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