STC89C53RC datasheet(148/165 Pages) STC

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All the usual rules for symbol names apply to module names. If a name is not provided, the module takes on

the file name (without a drive or subdirectory specifier and without an extension). In the absence of any use

of the NAME directive, a program will contain one module for each file. The concept of "modules," therefore,

is somewhat cumbersome, at least for relatively small programming problems. Even programs of moderate

size (encompassing, for example, several files complete with relocatable segments) needn't use the NAME

directive and needn't pay any special attention to the concept of "modules." For this reason, it was mentioned in

the definition that a module may be considered a "file," to simplify learning ASM51. However, for very large

programs (several thousand lines of code, or more), it makes sense to partition the problem into modules, where,

for example, each module may encompass several files containing routines having a common purpose.

Segment Selection Directives

When the assembler encounters a segment selection directive, it diverts the following code or data into the

selected segment until another segment is selected by a segment selection directive. The directive may select may

select a previously defined relocatable segment or optionally create and select absolute segments.

RSEG (Relocatable Segment)

The format for the RSEG (relocatable segment) directive is

RSEG

segment_name

Where "segment_name" is the name of a relocatable segment previously defined with the SEGMENT directive.

RSEG is a "segment selection" directive that diverts subsequent code or data into the named segment until another

segment selection directive is encountered.

Selecting Absolute Segments

RSEG selects a relocatable segment. An "absolute" segment, on the other

hand, is selected using one of the directives:

CSEG

(AT address)

DSEG

(AT address)

ISEG

(AT address)

BSEG

(AT address)

XSEG

(AT address)

These directives select an absolute segment within the code, internal data, indirect internal data, bit, or external

data address spaces, respectively. If an absolute address is provided (by indicating "AT address"), the assembler

terminates the last absolute address segment, if any, of the specified segment type and creates a new absolute

segment starting at that address. If an absolute address is not specified, the last absolute segment of the specified

type is continuted. If no absolute segment of this type was previously selected and the absolute address is omitted,

a new segment is created starting at location 0. Forward references are not allowed and start addresses must be

absolute.

Each segment has its own location counter, which is always set to 0 initially. The default segment is an

absolute code segment; therefore, the initial state of the assembler is location 0000H in the absolute code segment.

When another segment is chosen for the first time, the location counter of the former segment retains the last

active value. When that former segment is reselected, the location counter picks up at the last active value. The

ORG directive may be used to change the location counter within the currently selected segment.

ASSEMBLER CONTROLS

Assembler controls establish the format of the listing and object files by regulating the actions of ASM51. For the

most part, assembler controls affect the look of the listing file, without having any affect on the program itself.

They can be entered on the invocation line when a program is assembled, or they can be placed in the source file.

Assembler controls appearing in the source file must be preceded with a dollor sign and must begin in column 1.

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