Zilog Z80380 - Manual (Page 28)

4-3
Z380 ™
U SER ' S M ANUAL Z ILOG
DC-8297-03
4.2.4 Direct Address (DA)
When Direct Address mode is used, the data processed is
at the location whose memory or I/O port address is in the
instruction.
Instruction Memory or
OPERATION I/O Port
ADDRESS
→
OPERAND
The operand value is the contents of the location whose
address is in the instruction.
Depending on the instruction, the operand specified by
DA mode is either in the I/O address space (I/O instruction)
or memory address space (all other instructions).
This mode is also used by Jump and Call instructions to
specify the address of the next instruction to be executed.
(The address serves as an immediate value that is loaded
into the program counter.)
Also, DDIR Immediate Data Directives are used to expand
the direct address to 24 or 32 bits. Operand width is
affected by LW bit status for the load and exchange
instructions.
Example of DA mode:
1.
Load BC register from memory location 00005E22H in Word mode
LD BC, (5E22H) ;Load BC with the data in address
;00005E22H
BC
Before instruction execution 1234
After instruction execution 0301
Memory location 00005E22 01
00005E23 03
2. Load BC register from memory location 12345E22H in Word mode
DDIR IW ;extend direct address by one word
LD BC, (12345E22H) ;Load BC with the data in address
;12345E22H
BC
Before instruction execution 1234
After instruction execution 0301
Memory location 12345E22 01
12345E23 03
3. Load BC register from memory location 12345E22H in Long Word mode
DDIR IW,LW ;extend direct address by one word,
;and operation in Long Word
LD BC, (12345E22H) ;Load BC with the data in address
;12345E22H
BCz BC
Before instruction execution 1234 5678
After instruction execution 0705 0301
Memory location 12345E22 01
12345E23 03
12345E24 05
12345E25 07