User Area¶
The user area is a small RAM region for task local non-stack storage. It is basically used like variables.
The address of an user variable is built from two numbers: the user
area base address and an offset. The effective address is the sum
of both numbers. The user area base address is stored
in the user pointer UP. It can read with the up@ operation and can
be modified with the up! with immediate effect.
The user pointer is different for each task. The user area size is a parameter for task creation.
The offset handled by the developer. There is a system defined area
starting at offset 0 that is used by various amforth words. The size
of this area may change from version to version. Each architecture
directory has a file user.inc that contains the current settings.
Customize¶
The default amforth built does not provide application specific user
area space. To get some, the application master file has to be changed
with a proper setting for the APPUSERSIZE variable. This setting
defines the bytes allocated in addition to the default system user
space.
With this new setting, new hexfiles need to be generated and uploaded.
Use¶
To create a user variable, the offset number and the name is required.
> 42 user answer
ok
> 44 user question
ok
> 48 user foo
This sequence alocates a 2 byte (1 cell) space for the user variable
answer, 4 bytes (2 cells) for the user variable question and
an unspecified number of bytes (since it’s the last definition) for
the user variable foo. the settings for the APPUSERSIZE shall
be at least cover the offset 49. That is is has to be at least
49 - SYSUSERSIZE.
The variables itself work with the normal RAM operations
> 42 answer !
ok
> answer @ u.
42 ok
> 2424. question 2!
ok
> foo c@
ok
Operations like fill work as well.
The file defer.frt adds support for defers in the user area. They
are used exactly like normal defers, but store the XT in the user area.
> 42 Udefer answer
ok
> ' ver is answer
ok
> answer
amforth 6.8 RV32IM ok
>
Some system words like emit, key? and source are in
fact user defers and can thus be redefined on a per task basis.
The file value.frt adds support for values in the user area. They
store their value in the user area (instead of the EEPROM an AVR8),
but behave identically. The definition requires both the intial
value and the offset:
> 0 42 Uvalue answer
ok
> answer u.
0 ok
> 42 to answer
ok
> answer u.
42 ok
>
See also