W78C438C_06 Datasheet, PDF (12/22 Pages) Winbond – 8-BIT MICROCONTROLLER

W78C438C
5.6 Programming Difference
The W78C438C is programmed in the same way as the W78C32, except that the external data RAM
is accessed by a "MOVX @Ri" instruction. To support address paging, there is an additional 8-bit SFR
"HB" (high byte), which is a nonstandard register, at address 0A1H. During execution of the "MOVX
@Ri" instruction, the contents of HB are output to AP6. The page address is modified by loading the
HB register with a new value before execution of the "MOVX @Ri" instruction. To read/write the HB
register, one can use the "MOV direct" instruction or "read-modify-write" instructions. The HB register
does not support bit-addressable instructions.
[Example].
MOV
MOV
MOVX
MOV
MOVX
R1, #0H
; R1 = 0.
0A1H, #0FFH ; HB contents FFH.
A, @R1
; Read the contents of external RAM location FF00H into
; Accumulator.
0A1H, #12H ; HB contents 12H.
@R1, A
; Copies the contents of Accumulator into external RAM
; location 1200H.
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