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CHAPTER 3 CPU ARCHITECTURE
User’s Manual U12978EJ3V0UD
52
3.4.2 Short direct addressing
[Function]
The memory to be manipulated in the fixed space is directly addressed with 8-bit data in an instruction word.
The fixed space is the 256-byte space FE20H to FF1FH where the addressing is applied. An internal high-
speed RAM and special function registers (SFR) are mapped at FE20H to FEFFH and FF00H to FF1FH,
respectively.
The SFR area (FF00H to FF1FH), where short direct addressing is applied, is a part of the whole SFR area.
In this area, ports which are frequently accessed in a program and a compare register of the timer/event
counter are mapped, and these SFRs can be manipulated with a small number of bytes and clocks.
When 8-bit immediate data is at 20H to FFH, bit 8 of an effective address is set to 0. When it is at 00H to
1FH, bit 8 is set to 1. See [Illustration] below.
[Operand format]
Identifier Description
saddr Label or FE20H to FF1FH immediate data
saddrp Label or FE20H to FF1FH immediate data (even address only)
[Description example]
MOV FE30H, #50H; When setting saddr to FE30H and the immediate data to 50H
Instruction code 1 1 1 1 0 1 0 1
00110000
01010000
Opcode
30H (saddr-offset)
50H (Immediate data)
[Illustration]
15
0
Short direct memory
Effective
address
1
111111
8
07
Opcode
saddr-offset
α
When 8-bit immediate data is 20H to FFH, α = 0.
When 8-bit immediate data is 00H to 1FH, α = 1.