Zilog Z80380 - Manual (Page 133)

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Z380 ™
U SER ' S M ANUAL Z ILOG
DC-8297-03
LDDRW
LOAD, DECREMENT AND REPEAT (WORD)
LDDRW
Operation: repeat until (BC=0) begin
if (LW) then begin
(DE)
←
(HL)
(DE+1)
←
(HL+1)
(DE+2)
←
(HL+2)
(DE+3)
←
(HL+3)
DE
←
DE – 4
HL
←
HL – 4
BC(15-0)
←
BC(15-0) – 4
end
else begin
(DE)
←
(HL)
(DE+1)
←
(HL+1)
DE
←
DE – 2
HL
←
HL – 2
BC(15-0)
←
BC(15-0) – 2
end
end
This instruction is used for block transfers of strings of data. The words of data at the location
addressed by the HL register are loaded into memory starting at the location addressed by
the DE register. The number of words moved is determined by the contents of the BC
register. If the BC register contains zero when this instruction is executed, 65,536 words are
transferred. The effect of decrementing the pointers during the transfer is important if the
source and destination strings overlap with the source string starting at a lower memory
address. Placing the pointers at the highest address of the strings and decrementing the
pointers ensures that the source string is copied without destroying the overlapping area.
This instruction can be interrupted after each execution of the basic operation. The Program
Counter value of the start of this instruction is saved before the interrupt request is
accepted,so that the instruction can be properly resumed.
Flags: S: Unaffected
Z: Unaffected
H: Cleared
V: Cleared
N: Cleared
C: Unaffected
Addressing Execute
Mode Syntax Instruction Format Time Note
LDDRW 11101101 11111000 nX(3+r+w) L