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Mid-Range PIC ? Microcontroller Architectural Overview
2.4
ASSEMBLER BASICS
Numbers in the Assembler
Unless otherwise specified, the assembler assumes any numeric constants in the
program are hexadecimal (base 16). Binary (base 2), Octal (base 8), Decimal (base
10), and ASCII coding are also supported.
Hexadecimal
Decima l
Octal
Binary
ASCII
12 or 0x12 or H'12 '
.12 or D'12'
O'12'
B'00010010'
A'c' or 'c'
Org (Origin)
Org tells the Assembler an address at which to start generating code. Normally
we start coding at the Reset vector address ‘ 0000 ’, but it could be anywhere.
Baseline devices have a Reset vector at the last location in program memory, so
it’s good practice to have a GOTO instruction pointing to the beginning of the
program.
End
End tells the assembler to stop assembling. There must be one at the end of the
program. It does not necessarily have to be at the end of the file, but nothing after
the end statement will be assembled.
Defining Data Memory Locations
There are three ways to name a location (see Example 2-1). All are equivalent in
that the location name label will be substituted with the value assigned to it during
assembly.
EXAMPLE 2-1:
DEFINING DATA MEMORY
#define Length
0x20
;c-like syntax
Length
equ
0x20
;equate 0x20 with the symbol
cblock
Length
Width
Area:2
Girth
0x20
;start a block of variables
;this will be at address 0x20
;this will be at address 0x21
;this is 2 bytes long, starting at
;address 0x22
;this will be at address 0x24
endc
Note that if used as a literal, the label names will take on the value assigned. If used as
an address operand in an instruction, the label names point to the contents of the file
register with the address of the label’s value.
Unless there is a reason to name a specific location address, the cblock/endc
method is preferred. The advantage is that as variables come and go through the
development process, the cblock keeps the block to a minimum. Using one of the
other methods, you may have to go back and find an unused location.
? 2007 Microchip Technology Inc.
DS41296B-page 11
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