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Add DW8051 MPAGE SFR

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This adds the MPAGE (sdcc _XPAGE) SFR, which is used by sdcc in `--xstack`
mode to provide the upper byte of the address for `MOVX A, @Ri` and
`MOVX @Ri, A` instructions.

Classic 8051 uses the P2 SFR value for this, but the RTL IP block does not
seem to be using the "Port 2 module".

From the databook:
```
To replace the function of the Port 2 latch in designs that do not use a
Port 2 module, the DW8051 provides an additional special function register,
MPAGE, at SFR address 92h. During MOVX A, @Ri and MOVX @Ri, A instructions,
the DW8051 places the contents of the MPAGE register on the upper 8 address
bits (mem_addr[15:8]). This provides the paging function that is normally
provided by the Port 2 latch. The MPAGE register has no function when a
Port 2 module is used to connect external RAM.
```
This commit is contained in:
Tobias Diedrich
2026-03-01 09:32:13 +01:00
parent 23e2f5a99e
commit c3a6a46852
1 changed files with 3 additions and 0 deletions
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rtl837x_sfr.h
+3
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@@ -40,6 +40,9 @@ __sfr __at(0x91) EXIF;
__sfr __at(0xf8) EIP;
__sbit __at(0xf9) PX3;
/* MPAGE page (sdcc _XPAGE) register for "movx @Ri" */
__sfr __at(0x92) _XPAGE; /* _XPAGE is used by sdcc in xstack mode */
__sfr __at(0x92) MPAGE;
/* SFR Bank control register: 0x0-3f. A value of 0 is bank 1 */
__sfr __at(0x96) PSBANK;
// SFR used to store return bank for trampoline