Msg: 5626 *Conference*

01-05-95 16:33:56

From: RON WIESEN

To : COMET _

Subj: REPLY TO MSG #5618 (F647 (63047))

F647h/63047/-02489 is a temporary holding cell used when an Escape X sequence
(027,088) for the Liquid Crystal Display (LCD) is processed.  In the process,
the content of F647h/63047/-02489 is copied to F646h/63046/-02490 and then
F647h/63047/-02489 is cleared.  The code that does this is shown below.
 
444Ah/17482/-48054  LXI H,  F647h
                    MOV A,  M
                    MVI M,  0         ;clear holding cell
                    DCX H             ;HL=F646h/63046/-02490
                    MOV M,  A
                    RET
 
Linkage to 444Ah/17482/-48054 is in the last 3-byte entry of the LCD escape
sequence lookup table.  This entry is shown below.
 
43F7h/17399/-48137  DB      'X'
                    DW      444Ah     ;link address for Escape X sequence
 
I would guess that escape sequence processing for the LCD, and normal LCD
processes, are tied to the 4.0 millisecond clock interrupt (RST 7.5) handler.
So unless your REAL FAST, the content of the holding cell will become cleared
before you get a chance to PEEK it.  This is a guess, but you could verify this
with some m/l code that disables interrupts and then alters and inspects the
holding cell.