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Expressions are assigned classes based on the operands that are used. The

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following classes apply to expressions:

Expression Class

Description

N NUMB

A classless number.

C ADDR

A CODE address symbol.

D ADDR

A DATA address symbol.

I ADDR

An IDATA address symbol.

X ADDR

An XDATA address symbol.

B ADDR

A BIT address symbol.

CO ADDR

A CONST address symbol.

EC ADDR

An ECONST address symbol.

CE ADDR

An ECODE address symbol.

ED ADDR

An EDATA address symbol.

EB ADDR

An EBIT address symbol.

HD ADDR

An HDATA address symbol.

HC ADDR

An HCONST address symbol.

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Chapter 3. Writing Assembly Programs

1.

The result of a unary operation has the same class as its operand.

2.

The result of all binary operations except + and – will be a NUMBER type.

3.

If only one of the operands of an addition or subtraction operation has a class,

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the result will have that class. If both operands have a class, the result will be

a NUMBER.

data_address - 10

gives a data_address value

10 + edata_address

gives an edata_address value

(data_address - data_address)

gives a classless number

code_address + (data_address - data_address)

gives a code_address value

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Relocatable Expressions

Relocatable expressions are so named because they contain a reference to a

relocatable or external symbol. These types of expressions can only be partially

calculated by the assembler since the assembler does not know the final location

of relocatable segments. The final calculations are performed by the linker.

A relocatable expression normally contains only a relocatable symbol, however,

it may contain other operands and operators as well. A relocatable symbol can

be modified by adding or subtracting a constant value.

Examples for valid relocatable expression

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relocatable_symbol + absolute_expression

relocatable_symbol - absolute_expression

absolute_expression + relocatable_symbol

There are two basic types of relocatable expressions: simple relocatable

expressions and extended relocatable expressions.

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Chapter 3. Writing Assembly Programs

1.

As an operand to the ORG directive.

2.

As an operand to a symbol definition directive (i.e. EQU, SET)

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3.

As an operand to a data initialization directive (DB, DW or

DD)

4.

As an operand to a machine instruction

Examples for simple relocatable expressions

REL1 + ABS1 * 10

REL2 - ABS1

REL1 + (REL2 - REL3)

assuming REL2 and REL3 refer to the same segment.

Invalid form of simple relocatable expressions

(REL1 + ABS1) * 10

relocatable value may not be multiplied.

(EXT1 - ABS1)

this is a general relocatable expression

REL1 + REL2

you cannot add relocatable symbols.

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REL1 + ABS1 * 10

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EXT1 - ABS1

LOW (REL1 + ABS1)

WORD2 (SEG1)

Invalid form of simple relocatable expressions

(SEG1 + ABS1) * 10

relocatable value may not be multiplied.

(EXT1 - REL1)

you can add/subtract only absolute quantities

LOW (REL1) + ABS1

LOW may be applied only to the

final relocatable expression

EXTRN CODE (CLAB)

; entry in CODE space

EXTRN DATA (DVAR)

; variable in DATA space

MSK

EQU

0F0H

; define a symbol to replace 0xF0

VALUE

EQU

MSK - 1

; another constant symbolic value

LVAL

EQU

12345678H

; LVAL get the value 12345678H

?PR?FOO SEGMENT CODE

RSEG

?PR?FOO

ENTRY:

MOV

A,#40H

; load register with constant

MOV

R5,#VALUE

; load constant symbolic value

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MOV

R3,#(0x20 AND MASK)

; examples for calculations

MOV

R7,#LOW (VALUE + 20H)

MOV

R6,#1 OR (MSK SHL 4)

MOV

R0,DVAR+20

; load content from address DVAR+20

MOV

R1,#LOW (CLAB+10)

; load low byte of address CLAB+10

MOV

WR4,#WORD2 (LVAL)

; load high word of LVAL

MOV

DR0,#ENTRY

; load low word of addr. ENTRY to DR0

MOVH

DR0,#WORD2 (ENTRY)

; load high word of addr. ENTRY to DR0

MOV

R4,@DR0

; load content of ENTRY to R4

;

MOV

R5,80H

; load DATA addr. 80H (= SFR P0) to R5

MOV

R5,EDATA 80H

; load EDATA address 80H to R5

SETB

30H.2

; set bit at 30H.2 (long address)

SETB

20H.2

; set bit at 20H.2 (short address)