On the correct processing of R / I / A
VfNG / NEW
In many programs, people may want to grab a standard message
DOS Error reading / writing sector to
Lovely open its window, do
Request Retry / Abort / Ignore, get an answer
and the right to continue or terminate the program. Not all are
able to do it. I will try to illuminate this issue.
At # 3F0A DOS is a procedure that provides a record of the
sector, and by # 3F0E-reading. Is precisely because of these
procedures and is read / write standard features. Consider these
procedures carefully:
# 3F0A LD A, # A0; code write command
; Sector
JR # 3F10
# 3F0E LD A, # 80; code read command
; Sector
# 3F10 LD (# 5CFE), A
LD D, # 0A; number of attempts
; Reading / writing
# 3F15 PUSH DE
DI
LD A, (# 5CFF)
INC A
OUT (# 5F), A; sector number
LD HL, (# 5D00); buffer
LD C, # 7F; data port
LD A, (# 5CFE); command code
OUT (# 1F), A; invoking
CP # A0; entry?
PUSH AF
CALL Z, # 3FBA; call recording
; Sector
POP AF
CALL NZ, # 3FD5; reading challenge
; Sector
POP DE
EI
IN A, (# 1F); reading register
; State
LD B, A
AND # 7F
RET Z; out if there is no
; Error
LD HL, # 29D8; address labels
; "Read Only"
AND # 40; write protection?
JR NZ, # 3F4B; yes
LD A, B
AND 4, loss of data?
JR Z, # 3FA0; not. re
; Attempt to read.
DEC D; last attempt?
JR NZ, # 3F15; not, another
LD HL, # 29E2; address labels
; "Disk error"
LD A, # D0
OUT # 1F, A; cease to
; Mande, if it
, Continues to be executed
LD A, B
AND 1
JP NZ, # 3EE7; treatment
; "No disk"
IN A, # 3F; reading register
; Track
OR A; zero?
JR NZ, # 3F5F; yes
IN A, # 5F; reading register
; Sector
CP # 0A; ten?
RET Z; output, if so
# 3F5F PUSH HL
# 3F60 CALL # 1D97; call clearing
; Screen
POP HL
RST # 18
LD HL, # 2A13; address labels
; "Trk."
RST # 18; output
IN A, # 3F; reading register
; Track
CALL # 1DA3; withdrawal rates
; Track
LD HL, # 2A18; address labels
; "Sec."
RST # 18; output
IN A, # 5F; reading register
; Sector
CALL # 1DA3; withdrawal rates
; Sector
LD HL, # 29FE; address labels
; "Retry / Abort / Ignore?"
RST # 18; output labels
# 3F7B CALL # 1052, # 1052 procedure
And returns to the A ASCII-code pressed on the keyboard
# 3F7E CP "I"; Ignore?
RET Z; refund if
Yea
CP "R"; Retry?
JR Z, # 3F94; yes, access to
; The next cycle
; Attempts
CP "A"; Abort?
JR NZ, # 3F7B; not, go to
, Reading the keyboard
; Tours
# 3F89; Abort
The main thing that can be extracted from the drawn above,
is that as a result of error, we find ourselves either in the #
3F60, or # 3EE7. # 3F60 Poprubuem examine more closely.
# 3F60 CALL # 1D97
# 1D97 RST # 20
DEFB # 6B
DEFB # 0D
# 0020 JP # 2F72
# 2F72 LD (# 5D02), HL
LD (# 5D04), DE
POP HL
LD E, (HL)
INC HL
LD D, (HL)
INC HL
PUSH HL
LD HL, # 3D2F
PUSH HL
PUSH DE
LD HL, # 5CC2
PUSH HL
LD HL, (# 5D02)
LD DE, (# 5D04)
RET
Thus, when a disk error is different from "No disk", we fly
to # 5CC2. In # 5CC2 standard sticks RET. This point
used to call subroutines ROM
BASIC. Two bytes in # 5CC3 and # 5CC4 not used, then we are
legally able to enter in the # 5CC2 team of 3 bytes: JP
kudanibud, such as driver of treatment.
Note that after this most at JP
stack will protrude number # 0D6B. Hence,
make the following veshchichki:
DRIVER EX (SP), HL
PUSH AF
LD A, H
CP # 0D
JR Z, ERROR
POP AF
EX (SP), HL
RET
Guess for yourself what happened then. Well
and now
ERROR POP HL
POP HL
POP HL
POP HL
POP HL
CALL PROC; call their own
; RIA procedure return-to A "R", "I", "A"
, Respectively.
LD HL, # 3F7E
EX (SP), HL
JP # 3D2F; return to DOS
; Answer!
In the simplest case CALL PROC replaced by LD A, "R" / "I" /
"A", if the answer to error is defined odnaznachno.
Similarly, although somewhat more complicated
is the case with the processing of "No disk"
(# 3EE7). In this, as well as to depart on
"Abort", sort it out themselves, it is not difficult
do.
