Short Direct Addressing - NEC mPD178054 Series User Manual

3.4.4 Short direct addressing

[Function]
The memory to be manipulated in the fixed space is directly addressed with 8-bit data in an instruction word.
This addressing is applied to the fixed 256-byte space FE20H to FF1FH. An internal RAM and a special-function
register (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 one part of all the SFR areas. In
this area, ports which are frequently accessed in a program and a compare register and a capture 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. Refer to [Illustration] below.
[Operand format]
Symbol
saddr
saddrp
[Example]
MOV 0FE30H, #50H; when setting saddr to FE30H and immediate data to 50H
Operation code
[Illustration]
7
15
Effective address
1 1
When 8-bit immediate data is 20H to FFH, α = 0
When 8-bit immediate data is 00H to 1FH, α = 1
CHAPTER 3 CPU ARCHITECTURE
Label of FE20H to FF1FH immediate data
Label of FE20H to FF1FH immediate data (even address only)
0 0 0 1 0 0 0 1
0 0 1 1 0 0 0 0
0 1 0 1 0 0 0 0
0
OP code
saddr-offset
8 7
α
1 1 1 1 1
User's Manual U15104EJ2V0UD
Description
Op code
30H (saddr-offset)
50H (immediate data)
0
Short direct memory
63