STC89LE52RC datasheet(138/165 Pages) STC

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Generic Jumps and Calls

ASM51 allows programmers to use a generic JMP or CALL mnemonic. "JMP" can be used instead of SJMP,

AJMP or LJMP; and "CALL" can be used instead of ACALL or LCALL. The assembler converts the generic

mnemonic to a "real" instruction following a few simple rules. The generic mnemonic converts to the short form

(for JMP only) if no forward references are used and the jump destination is within -128 locations, or to the

absolute form if no forward references are used and the instruction following the JMP or CALL instruction is in

the same 2K block as the destination instruction. If short or absolute forms cannot be used, the conversion is to

the long form.

The conversion is not necessarily the best programming choice. For example, if branching ahead a few

instrucions, the generic JMP will always convert to LJMP even though an SJMP is probably better. Consider the

following assembled instructions sequence using three generic jumps.

LOC

OBJ

LINE

SOURCE

1234

ORG

1234H

1234

START:

INC

1235

80FD

JMP

START

;ASSEMBLES AS SJMP

12FC

ORG

START + 200

12FC

4134

JMP

START

;ASSEMBLES AS AJMP

12FE

021301

JMP

FINISH

;ASSEMBLES AS LJMP

1301

FINISH:

INC

END

The first jump (line 3) assembles as SJMP because the destination is before the jump ( i.e., no forward reference)

and the offset is less than -128. The ORG directive in line 4 creates a gap of 200 locations between the label

START and the second jump, so the conversion on line 5 is to AJMP because the offset is too great for SJMP.

Note also that the address following the second jump (12FEH) and the address of START (1234H) are within the

same 2K page, which, for this instruction sequence, is bounded by 1000H and 17FFH. This criterion must be met

for absolute addressing. The third jump assembles as LJMP because the destination (FINISH) is not yet defined

when the jump is assembled (i.e., a forward reference is used). The reader can verify that the conversion is as

stated by examining the object field for each jump instruction.

ASSEMBLE-TIME EXPRESSION EVALUATION

Values and constants in the operand field may be expressed three ways: (a) explicitly (e.g.,0EFH), (b) with a

predefined symbol (e.g., ACC), or (c) with an expression (e.g.,2 + 3). The use of expressions provides a powerful

technique for making assembly language programs more readable and more flexible. When an expression is used,

the assembler calculates a value and inserts it into the instruction.

All expression calculations are performed using 16-bit arithmetic; however, either 8 or 16 bits are inserted

into the instruction as needed. For example, the following two instructions are the same:

MOV

DPTR,

#04FFH + 3

MOV

DPTR,

#0502H

;ENTIRE 16-BIT RESULT USED

If the same expression is used in a "MOV A,#data" instruction, however, the error message "value will not fit in a

byte" is generated by ASM51. An overview of the rules for evaluateing expressions follows.

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