diff --git a/auxmem.hostfs.s b/auxmem.hostfs.s
index 48b7dd8..c3b9301 100644
--- a/auxmem.hostfs.s
+++ b/auxmem.hostfs.s
@@ -60,22 +60,31 @@ OSFINDRET    >>>   ENTAUX
              RTS
 
 * OSGBPB - Get/Put a block of bytes to/from an open file
-GBPBHND      PHX
+* Supports commands A=1,2,3,4 others unsupported
+GBPBHND      CMP   #4
+             BCC   :S1
+             RTS
+:S1          PHX
              PHY
              PHA
+             >>>   WRTMAIN
+             STX   GBPBAUXCB+0               ; Copy address of control block ..
+             STY   GBPBAUXCB+1               ; .. to main mem
+             >>>   WRTAUX
              JSR   XYtoLPTR                  ; Copy control block to GBPBBLK ..
              LDY   #$0C                      ; .. in main memory
              >>>   WRTMAIN
 :L1          LDA   (OSLPTR),Y
              STA   GBPBBLK,Y
              DEY
-             BNE   :L1
+             BPL   :L1
              >>>   WRTAUX
-             PLA                             ; A represents the command
+             PLA                             ; A => OSGBPB command
              >>>   XF2MAIN,GBPB
 OSGBPBRET    >>>   ENTAUX
              PLY
              PLX
+             LDA   #$00                      ; A=0 means supported command
              RTS
 
 * OSBPUT - write one byte to an open file
diff --git a/mainmem.fsequ.s b/mainmem.fsequ.s
index fb4f610..d3c73b0 100644
--- a/mainmem.fsequ.s
+++ b/mainmem.fsequ.s
@@ -22,8 +22,9 @@ GBPBHDL     EQU   GBPBBLK+0   ; File handle
 GBPBDAT     EQU   GBPBBLK+1   ; Pointer to data (4 bytes)
 GBPBNUM     EQU   GBPBBLK+5   ; Num bytes to transfer (4 bytes)
 GBPBPTR     EQU   GBPBBLK+9   ; File pointer (4 bytes)
+GBPBAUXCB   EQU   GBPBBLK+13  ; Address of aux control block (2 bytes)
 
-*                 $038F       ; $2F bytes here
+*                 $0391       ; $2D bytes here
 
 * Control block for OSFILE
 FILEBLK     EQU   $03BE
diff --git a/mainmem.svc.s b/mainmem.svc.s
index f638996..d6160f9 100644
--- a/mainmem.svc.s
+++ b/mainmem.svc.s
@@ -327,8 +327,46 @@ CFILE         >>>   ENTMAIN
 * A=2 : Write bytes to disk, ignoring seq pointer
 * A=3 : Read bytes from disk, using new seq pointer
 * A=4 : Read bytes from disk, ignoring seq pointer
+* All others unsupported
 GBPB          >>>   ENTMAIN
 *             ...
+              LDY   GBPBHDL            ; File ref number
+              STY   READPL2+1
+:L1           JSR   MLI                ; Read one byte
+              DB    READCMD
+              DW    READPL2
+              BCS   :ERR
+              LDA   GBPBDAT+0
+              STA   ZPMOS+0
+              LDA   GBPBDAT+1
+              STA   ZPMOS+1
+              LDA   BLKBUF
+              >>>   WRTAUX
+              STA   (ZPMOS)            ; Store byte in aux mem
+              >>>   WRTMAIN
+              INC   GBPBDAT+0          ; Increment data pointer
+              BNE   :S1
+              INC   GBPBDAT+1
+:S1           LDA   GBPBNUM+0          ; Decrement number of bytes
+              BNE   :S2
+              LDA   GBPBNUM+1
+              BEQ   :ZERO              ; Zero remaining, done!
+              DEC   GBPBNUM+1
+:S2           DEC   GBPBNUM+0
+              BRA   :L1
+*             ...
+:ERR
+:ZERO         LDA   GBPBAUXCB+0        ; Copy control block back ..
+              STA   ZPMOS+0            ; .. to aux memory
+              LDA   GBPBAUXCB+1
+              STA   ZPMOS+1
+              LDY   #$0C+1
+              >>>   WRTAUX
+:L2           LDA   GBPBBLK,Y
+              STA   (ZPMOS),Y
+              DEY
+              BPL   :L2
+              >>>   WRTMAIN
               >>>   XF2AUX,OSGBPBRET
 
 * ProDOS file handling for MOS OSBGET call