diff --git a/blk.fs b/blk.fs
index 9d88b9e..1a7feb4 100644
--- a/blk.fs
+++ b/blk.fs
@@ -1078,6 +1078,7 @@ HERESTART [IF]
 [THEN]
     SYSVARS 0x04 + LD(i)HL, ( RAM+04 == HERE )
     A XORr, SYSVARS 0x3e + LD(i)A, ( 3e == ~C! )
+    SYSVARS 0x41 + LD(i)A, ( 41 == ~C!ERR )
     DE BIN( @ 0x04 ( BOOT ) + LDd(i),
     JR, L1 FWR ( execute, B287 )
 ( ----- 286 )
@@ -1689,7 +1690,7 @@ driver code. Load the low part with "353 LOAD", the high part
 with "390 LOAD"
 ( ----- 353 )
 : RAM+ [ SYSVARS LITN ] + ; : BIN+ [ BIN( @ LITN ] + ;
-: HERE 0x04 RAM+ ;
+: HERE 0x04 RAM+ ; : ~C!ERR 0x41 RAM+ ;
 : CURRENT* 0x51 RAM+ ; : CURRENT CURRENT* @ ;
 : H@ HERE @ ;
 : FIND ( w -- a f ) CURRENT @ SWAP _find ;
@@ -2159,14 +2160,18 @@ XCURRENT @ _xapply ORG @ 0x04 ( stable ABI BOOT ) + !
 ':' X' _ 4 - C! ( give : its name )
 '(' X' _ 4 - C!
 ( ----- 400 )
-( Write byte E at addr HL, assumed to be an AT28 EEPROM.
-  After that, poll repeatedly that address until writing is
-  complete. )
+( Write byte E at addr HL, assumed to be an AT28 EEPROM. After
+  that, poll repeatedly that address until writing is complete.
+  If last polled value is different than orig, set ~C!ERR )
 (entry) ~AT28 ( warning: don't touch D register )
-    (HL) E LDrr, E (HL) LDrr, ( poll ) BEGIN,
+    (HL) E LDrr, A E LDrr, ( orig ) EXAFAF', ( save )
+    E (HL) LDrr, ( poll ) BEGIN,
         A (HL) LDrr, ( poll ) E CPr, ( same as old? )
         E A LDrr, ( save old poll, Z preserved )
-    JRNZ, AGAIN, RET,
+    JRNZ, AGAIN,
+    EXAFAF', ( orig ) E SUBr, ( equal? )
+    IFNZ, SYSVARS 0x41 + ( ~C!ERR ) LD(i)A, THEN,
+    RET,
 ( ----- 401 )
 Grid subsystem
 
diff --git a/cvm/stage.bin b/cvm/stage.bin
index 6827b77..cb79ff7 100644
Binary files a/cvm/stage.bin and b/cvm/stage.bin differ
diff --git a/doc/impl.txt b/doc/impl.txt
index ca25816..eaeb521 100644
--- a/doc/impl.txt
+++ b/doc/impl.txt
@@ -160,9 +160,9 @@ Here's a list of them:
 
 SYSVARS   FUTURE USES          +3c       BLK(*
 +02       CURRENT              +3e       ~C!*
-+04       HERE                 +41       FUTURE USES
-+06       C<?                  +43       FUTURE USES
-+08       C<* override         +45       FUTURE USES
++04       HERE                 +41       ~C!ERR
++06       C<?                  +42       FUTURE USES
++08       C<* override         +43       FUTURE USES
 +0a       NL ialias            +51       CURRENTPTR
 +0c       C<*                  +53       EMIT ialias
 +0e       WORDBUF              +55       KEY ialias
@@ -197,6 +197,10 @@ BLK* "Disk blocks" in usage.txt.
 the routine that writes a byte to memory and then returns.
 Register usage is arch-dependent, see boot code for details.
 
+~C!ERR: When an error happens during ~C! write overrides, sets
+this byte to a nonzero value. Otherwise, stays at zero. Has to
+be reset to zero manually after an error.
+
 DRIVERS section is reserved for recipe-specific drivers.
 
 FUTURE USES section is unused for now.          
@@ -214,7 +218,8 @@ very few things.
 2. Set CURRENT to value of LATEST field in stable ABI.
 3. Set HERE to HERESTART const if defined, to CURRENT other-
    wise.
-4. Execute the word referred to by 0x04 (BOOT) in stable ABI.
+4. Initialize ~C! and ~C!ERR to 0.
+5. Execute the word referred to by 0x04 (BOOT) in stable ABI.
 
 In a normal system, BOOT is in core words at B396 and does a
 few things: