doc: update AT28 instructions for new ~AT28 routine

doc/hw/at28.txt

@@ -31,14 +31,31 @@ I don't think you need a schematic. It's really simple.
 
 # Writing contents to the AT28
 
-There is an AT28! writer word in B400 which is C!*-compatible
-(see "Indirect memory access" in doc/usage.txt) and and waits
-until the write is complete before returning. If you use C!
-directly, bytes you write might not have the time to completely
-write themselves before you write another one.
-
-To use, set C!* with "' AT28! ' C!* **!". Once this is done, you
-can use MOVE, or /tools/upload, both will call C!* and do the
-right thing. Unset C!* with "' C! ' C!* **!" afterwards.
-
-B400 also has AT28, which is the same as AT28!, but for C,.
+If you wait 10ms between each byte you write, you can write dir-
+ectly to the AT28 with regular memory access words. If you don't
+wait, the AT28 writing program will fail. Because it's not very
+pratical to insert waiting time between each byte writes, you
+need another solution.
+
+To that end, Collapse OS has a "memory write override" mech-
+anism. Whenever C! or ! is about to set a byte somewhere in
+memory, it checks whether such an override is active. If it is,
+it calls it. That override is set with the "~C!" word.
+
+B400 contains an override routine compatible with ~C! called
+~AT28. When you're about to write to your AT28, activate that
+override with "' ~AT28 ~C!". That overwrite will write the byte,
+then poll the AT28 until it indicates that it is finished
+writing. This ensures that Collapse OS doesn't try writing
+another byte before the AT28 is ready.
+
+When you're done writing to the AT28, unset override with
+"0 ~C!". The override routine has a non-negligible speed impact
+on all memory writes.
+
+When polling, ~AT28 also verifies that the final byte in memory
+is the same as the byte written. If it's not, it will place a
+non-zero value in the ~C!ERR 1b variable. Therefore, if you want
+to see, after a big write operation to your AT28, whether any
+write failed, do "~C!ERR C@ .". Re-initialize to zero before
+your next write operation.