mirror of
https://git.sr.ht/~crc_/retroforth
synced 2024-11-16 19:48:56 +01:00
91 lines
2.6 KiB
Text
91 lines
2.6 KiB
Text
# Byte_Addressing
|
|
|
|
This adds words allowing access to memory at a byte level.
|
|
|
|
Nga is cell addressed. There's no direct access to memory at a byte
|
|
level. To access a specific byte one must fetch a cell, then extract
|
|
the individual bytes by masking and shifting. Updating bytes is
|
|
similarly annoying. The words here take care of these details.
|
|
|
|
It was originally developed for RETRO11 by Marc Simpson and has been
|
|
updated for RETRO12 by Charles Childers.
|
|
|
|
## Authors
|
|
|
|
* 2010, Marc Simpson
|
|
* 2011, 2018-2019, Charles Childers
|
|
|
|
## Functions
|
|
|
|
+-------------------+--------+----------------------------+
|
|
| Function | Stack | Used For |
|
|
+===================+========+============================+
|
|
| b:to-byte-address | a-a | Return the byte address of |
|
|
| | | the first byte in a cell |
|
|
+-------------------+--------+----------------------------+
|
|
| b:unpack | c-bbbb | Given a byte-packed cell |
|
|
| | | on the stack, return the |
|
|
| | | bytes it contains |
|
|
+-------------------+--------+----------------------------+
|
|
| b:pack | bbbb-c | Pack four byes into a cell,|
|
|
| | | return the cell on the |
|
|
| | | stack |
|
|
+-------------------+--------+----------------------------+
|
|
| b:fetch | a-b | Fetch a byte |
|
|
+-------------------+--------+----------------------------+
|
|
| b:store | ba- | Store a byte into memory |
|
|
+-------------------+--------+----------------------------+
|
|
|
|
## Code & Commentary
|
|
|
|
~~~
|
|
{{
|
|
'Byte var
|
|
|
|
:byte-mask (xn-b)
|
|
#255 swap #8 * dup
|
|
[ n:negate shift and ] dip shift ;
|
|
|
|
:replace
|
|
#0 [ [ [ [ drop @Byte ] dip ] dip ] dip ] case
|
|
#1 [ [ [ drop @Byte ] dip ] dip ] case
|
|
#2 [ [ drop @Byte ] dip ] case
|
|
#3 [ drop @Byte ] case
|
|
drop ;
|
|
|
|
---reveal---
|
|
|
|
:b:to-byte-address (a-a) #4 * ;
|
|
|
|
:b:unpack (c-bbbb)
|
|
dup #255 and swap
|
|
dup #8 shift #255 and swap
|
|
dup #16 shift #255 and swap
|
|
#24 shift #255 and ;
|
|
|
|
:b:pack (bbbb-c)
|
|
#-24 shift swap
|
|
#-16 shift + swap
|
|
#-8 shift + swap + ;
|
|
|
|
:b:fetch (a-b)
|
|
#4 /mod swap
|
|
#4 /mod
|
|
rot + fetch swap byte-mask ;
|
|
|
|
:b:store (ba-)
|
|
swap !Byte
|
|
#4 /mod swap [ dup fetch b:unpack ] dip
|
|
replace b:pack swap store ;
|
|
}}
|
|
~~~
|
|
|
|
|
|
## Some Tests
|
|
|
|
```
|
|
&s:length #4 * #0 + b:fetch
|
|
&s:length #4 * #1 + b:fetch
|
|
&s:length #4 * #2 + b:fetch
|
|
&s:length #4 * #3 + b:fetch
|
|
```
|