ZiLOG Z80-CPU Technical Manual page 40

Disable Interrupt
LDA,n
OUTn,A
Enable Interrupt
Return
prevent interrupt before
routine is exited.
notify peripheral that service
routine is complete
This seven byte sequence can be replaced with the one byte EI instruction and the two byte RETI instruction
in the l80. TItis is important since interrupt service time often must be minimized.
To facilitate program loop control the instruction DJNl e can be used advantageously. This two byte,
relative jump instruction decrements the B register and the jump occurs if the B register has not been decre-
mented to zero. The relative displacement is expressed as a signed two's complement number. A simple ex-
ample of its use nlight be:
Address
N, N+ 1
N + 2 to N+ 9
N + 10, N+ 11
N+ 12
JUMP 'JP'
.JUMP
6JR'
JUMP 'JP'
JUMP 'JP'
JUMP
'JP'
'CALL'
DECREMENT B,
JUMP IF NON
ZERO'DJNZ'
RETURN
'RET'
Instruction
LDB, 7
(perform a sequence
of instructions)
DJNl -8
(Next Instruction)
IMMED.
nn
EXT.
RELATIVE
PC+e
(HL)
REG.
(IX)
INDIR.
(IV)
IMMED. nn
EXT.
RELATIVE
PC+e
REGISTER
(SP)
INDIR.
(SP+l)
RETURN FROM REG.
(SP) ED
(SP+l) 4D INT'RET/' INDIR.
RETURN FROM
NON MASKABLE REG.
INT'RETN' INDIR.
ED
(SP+l) 45
Comments
; set B register to count of 7
; loop to be performed 7 times
; to jump from N + 12 to N + 2
CONDITION
28
;w
e-2 e-2
NOTE-CERTAIN
FLAGS HAVE MORE
THAN ONE PURPOSE.
REFER TO SECTION
6_0 FOR DETAILS
JUMP, CALL and RETURN GROUP
TABLE 5.3-11
34