ZiLOG Z80 Handbook page 209

tion for the user. The interrupt processing routine would call the
subroutine with a user number.
LD
A,3
THIS IS USER # 3
CALL GETCH LOOK FOR NEXT CHARACTER
OR
A
SET FLAGS
JP NZ,AVAIL GO IF CHARACTER AVAILABLE
The subroutine at GETCH would utilize the user number to find
the predefined user area ( or task block) for storage of data and vari-
ables if required.
GETCH PUSH AF SAVE USER #
SLA A ( USER #) * 2
LD C,A ( USER #) * 2 IN BC
LD B,0
LD HL , BUFTB TABLE OF BUFFER ADDRESSES
ADD HL,BC
LD E,(HL)
INC HL
LD D,(HL) GET BUFFER ADDRESS
CALL READK READ KEYBOARD
JP Z,OUT GO IF NONE AVAILABLE
LD (HL,) A STORE CHARACTER
BUFTB DEFW BUFFO TABLE OF BUFFER
DEFW BUFF1 ADDRESSES USERS
DEFW BUFF2 0 THRU 4
DEFW BUFF3
DEFW BUFF4
Other ways to avoid the reentrancy problem are "lock - outs" of
subroutines in use, disabling of interrupts during critical sequences,
and replication of code for each level of user, but the above tech-
niques are probably the most common and the use of stack for tem-
porary storage is the cleanest implementation.
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