Short Direct Addressing - NEC mPD780208 Subseries 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.
The fixed space to which this addressing is applied to is the 256-byte space from FE20H to FF1FH. An internal
high-speed RAM and special-function registers (SFRs) 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 total SFR area. In
this area, ports which are frequently accessed in a program and compare and capture registers 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]
Identifier
saddr
saddrp
[Description example]
MOV 0FE30H, #50H; when setting saddr to FE30H and immediate data to 50H
Operation code
[Illustration]
7
15
Effective address 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
Description
Label or immediate data indicating FE20H to FF1FH
Label or immediate data indicating FE20H to FF1FH (even address only)
0 0 0 1
0 0 1 1
0 1 0 1
0
Opcode
saddr-offset
8
α
1 1 1 1 1 1
User's Manual U11302EJ4V0UM
0 0 0 1 Opcode
0 0 0 0 30H (saddr-offset)
50H (immediate data)
0 0 0 0
Short direct memory
0
75