Propeller Manual

ORG – Assembly Language Reference
ORG
Directive: Adjust compile-time assembly pointer.
ORG 〈Address〉
• Address is an optional Cog RAM address (0-495) to assemble the following assembly
code with. If Address is not given, the value 0 is used.
Explanation
The
ORG (origin) directive sets the Propeller Tool’s assembly pointer to a new value for use in
address references within the assembly code to follow.
ORG typically appears as ORG 0, or just
ORG, at the start of any new assembly code intended to be launched into a separate cog.
ORG only affects symbol references; it does not affect the position of assembly code in the cog
itself. When assembly code is launched via a
COGNEW or COGINIT command, the destination
cog always loads the code into its RAM starting at address 0.
Even though assembly code is always loaded in this way, the compiler/assembler does not
know which part of the code constitutes its beginning since developers are free to launch any
code starting from any address.
To solve this, the assembler uses a reference point (the assembly pointer value) to calculate
the absolute address of referenced symbols. Those absolute addresses are encoded into the
assembly instruction’s destination (d-field) or source (s-field) in place of the symbolic
reference. For example:
DAT
org 0 'Start at Cog RAM 0
Toggle mov dira, Pin 'Set I/O direction to output
:Loop xor outa, Pin 'Toggle output pin state
jmp #:Loop 'Loop endlessly
Pin long $0000_0010 'Use I/O pin 4 ($10 or %1_0000)
The
ORG statement in this example sets the assembly pointer to zero (0) so the code following
it is assembled with that reference point in mind. Because of this, to the assembler the
Toggle
symbol is logically at Cog RAM location 0, the
:Loop symbol is at Cog RAM location 1, and
the
Pin symbol is at Cog RAM location 3. The assembler will replace each reference to these
symbols with their respective hard-coded location.
Page 328 · Propeller Manual v1.1