DC.W
Define Constant Directive
Format: DC.W < operand>
The function of this directive is to define a constant in memory. The DC.W directive can
have only one, 16 bit, operand which can contain the actual value (decimal, hexadecimal,
or ASCII). Alternatively, the operand may be an expression which can be assigned a nu-
meric value by the assembler. The constant is aligned on a word boundary as specified
by word (.W) size.
An ASCII string is recognised when characters are enclosed in single quotes (’). Each
character (7 bits) is assigned to a byte of memory, with the eighth bit (MSB) always
equal to zero. If one byte is entered, the byte is left justified.
Examples are:
DC.W
1234 Decimal number
DC.W
$AAFE Hexadecimal number
DC.W
’AB’ ASCII string
DC.W
’TB’+ 1 Expression
Entering and Modifying Source Programs
Application programs are entered into the PME 68-1B RAM area using the one-line
assembler/disassembler. It is entered in assembly language statements on a line-by-line
base. The source code is not saved as it is immediately converted to machine code. This
imposes several restrictions on the type of source line that can be entered. Labels and
symbols are not allowed; absolute values or addresses or PC relative offsets have to be
used.
Editing is accomplished by retyping the new source line. Lines can be added or deleted
by moving a block of memory data to free up or delete the appropriate number of
locations.
Calling the Assembler/Disassembler
The command MM< address> ; DI transfers control to the resident assembler/
disassembler. The contents of the specified locations are immediately disassembled if
they contain a valid instruction, otherwise a DC.W operand is displayed. The ? prompts
for a new input.
Example:
> MM 2000; DI < CR>
002000 0C000020 CMP.B # 32,D0 ?
If nothing is entered but < CR> , the next instruction is displayed.
PME68-1B Manual
Page 112 Issue 5
Содержание PME 68-1B
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