fix crash: 0x23CC50 is the RDP pointer, not a CSR0 shadow

The restore wrote 0x0042 into 0x23CC50, overwriting the driver's RDP pointer
(0x05800000); the next CSR access then targeted 0x0042 and the terminal died
with an illegal opcode. The TX kick does move @0x23CC50,a3 / move *a3,a5 --
it reads the live CSR0 and ORs TDMD, there is no shadow cell.

Report: section 10 corrected, section 62 added with the disassembly and the
consequence.

harness/mame/NETWORK_REPORT.md

@@ -249,10 +249,21 @@ a5 |= 0x0008                 ; TDMD
 > real-silicon `CSR0 = 0x1BB` decode (`STRT`=1, `TDMD`=1, `TXON`=1) shows a poll
 > that never ran, which is a different problem entirely.
 
-**The transmit demand is sent as `shadow | TDMD`, never as a bare `TDMD`.** The
-write must preserve the shadow's read/write bits (in particular `INEA`), and it
-matches the driver; our payload arms the descriptor at kick time and writes
-`STRT|TDMD` in one word.
+**The transmit demand is sent as `CSR0-read-back | TDMD`.** `0x23CC50` is **not**
+a shadow cell — it holds the driver's **RDP pointer** (`0x05800000`), and the kick
+reads CSR0 back through it:
+
+```
+3F9D5960  move @0x23CC50,a3,1    ; a3 = RDP pointer (0x05800000)
+3F9D5990  move *a3,a5,0          ; a5 = *RDP  = current CSR0
+3F9D59B0  or   a3=8,a5           ; a5 |= TDMD
+3F9D59D0  move @0x23CC30,a5,1    ; a5 = RDP pointer (second alias)
+3F9D5A00  move *a13,*a5,0        ; *RDP = a5
+```
+
+So the driver reads the *live* CSR0 and ORs `TDMD`, preserving whatever
+read/write bits (notably `INEA`) are currently set. Section 10 called that cell a
+"shadow" and section 62 records why that is wrong and what it broke.
 
 ## 11. The live driver paths (RX poll, TX completion)
 
@@ -4225,3 +4236,34 @@ from the real unit (they are the ones that displayed correctly there). MAME's
 frame dump does **not** reproduce the panel — the OS repaints the screen
 continuously there, and the real unit's VRAM layout differs slightly — so the
 panel is verified on the machine, not from an emulator screenshot.
+
+## 62. Correction: `0x23CC50` is the RDP pointer, not a CSR0 shadow
+
+Section 10 (and the restore code written from it) treated cell `0x23CC50` as a
+software shadow of CSR0 and wrote `0x0042` into it. That is wrong and it
+**crashed the terminal with an illegal opcode** on the machine.
+
+The disassembly is unambiguous:
+
+```
+3F9D5960  move @0x23CC50,a3,1    ; a3 = value of the cell = 0x05800000
+3F9D5990  move *a3,a5,0          ; a5 = *(0x05800000) = *RDP = current CSR0
+3F9D59B0  or   a3=8,a5           ; a5 |= TDMD
+3F9D59D0  move @0x23CC30,a5,1    ; a5 = second RDP alias (also 0x05800000)
+3F9D5A00  move *a13,*a5,0        ; *RDP = a5
+```
+
+`0x23CC50` holds a **pointer** (`0x05800000`, matching the RAM dump in section 2),
+and the driver reads the *live* CSR0 through it before OR-ing `TDMD`. There is no
+shadow: the "preserved" bits are simply whatever the chip currently reports.
+
+Consequences, all now fixed:
+
+* writing `0x0042` into `0x23CC50` overwrites the RDP pointer, so the driver's
+  next CSR access targets address `0x0042` — the fault observed on the unit;
+* the restore must write CSR0 through `RAP`/`RDP`, never through that cell;
+* section 10's phrase "CSR0 = shadow | TDMD" should read "CSR0 = read-back | TDMD".
+
+The restore path in `harness/udpstream/lance.c` now re-programs the chip with
+`0x7F41` (the firmware's own init value: `INIT | INEA | all status bits`), polls
+`IDON`, writes `0x0042`, and leaves the driver's cells untouched.

harness/udpstream/lance.c

@@ -408,8 +408,10 @@ int main()
                     break;
                 }
             }
-            /* keep the driver's command shadow consistent with the chip */
-            *(unsigned int *)0x23CC50 = 0x0042;
+            /* NOTE: do not write 0x23CC50 -- it is the driver's RDP pointer
+             * cell (0x05800000), not a CSR0 shadow.  The TX kick reads CSR0
+             * back through it and ORs TDMD; storing here corrupts the pointer
+             * and the next CSR access faults (illegal opcode). */
         }
         cell(0x260, csr0res);                   /* CSR0 after the restore */
         cell(0x280, osblk);                     /* init block we handed back */

harness/udpstream/lance_fixlen.c

@@ -408,8 +408,10 @@ int main()
                     break;
                 }
             }
-            /* keep the driver's command shadow consistent with the chip */
-            *(unsigned int *)0x23CC50 = 0x0042;
+            /* NOTE: do not write 0x23CC50 -- it is the driver's RDP pointer
+             * cell (0x05800000), not a CSR0 shadow.  The TX kick reads CSR0
+             * back through it and ORs TDMD; storing here corrupts the pointer
+             * and the next CSR access faults (illegal opcode). */
         }
         cell(0x260, csr0res);                   /* CSR0 after the restore */
         cell(0x280, osblk);                     /* init block we handed back */