diff --git a/client/inc/nb65_constants.i b/client/inc/nb65_constants.i
index b87b924..afeae16 100644
--- a/client/inc/nb65_constants.i
+++ b/client/inc/nb65_constants.i
@@ -34,7 +34,7 @@ NB65_UDP_ADD_LISTENER          EQU $06 ;inputs: AX points to a UDP listener para
 NB65_GET_INPUT_PACKET_INFO     EQU $07 ;inputs: AX points to a UDP packet parameter structure, outputs: UDP packet structure filled in
 NB65_SEND_UDP_PACKET           EQU $08 ;inputs: AX points to a UDP packet parameter structure, outputs: none packet is sent
 NB65_DEACTIVATE                EQU $09 ;inputs: none, outputs: none (removes call to NB65_VBL_VECTOR on IRQ chain)
-
+NB65_TFTP_CALLBACK_DOWNLOAD    EQU $0A ;inputs: AX points to a TFTP parameter structure, outputs: none
 NB65_PRINT_ASCIIZ              EQU $80 ;inputs: AX=pointer to null terminated string to be printed to screen, outputs: none
 NB65_PRINT_HEX                 EQU $81 ;inputs: A=byte digit to be displayed on screen as (zero padded) hex digit, outputs: none
 NB65_PRINT_DOTTED_QUAD         EQU $82 ;inputs: AX=pointer to 4 bytes that will be displayed as a decimal dotted quad (e.g. 192.168.1.1)
@@ -56,7 +56,7 @@ NB65_DRIVER_NAME     EQU $1A     ;2 byte pointer to name of driver
 ;offsets in TFTP parameter structure (used by NB65_TFTP_DIRECTORY_LISTING & NB65_TFTP_DOWNLOAD)
 NB65_TFTP_IP         EQU $00                     ;4 byte IP address of TFTP server
 NB65_TFTP_FILENAME   EQU $04                     ;2 byte pointer to asciiz filename (or filemask in case of NB65_TFTP_DIRECTORY_LISTING)
-NB65_TFTP_POINTER    EQU $06                     ;2 byte pointer to memory location data to be stored in
+NB65_TFTP_POINTER    EQU $06                     ;2 byte pointer to memory location data to be stored in OR address of callback function
 
 ;offsets in DNS parameter structure (used by NB65_DNS_RESOLVE)
 NB65_DNS_HOSTNAME    EQU $00                         ;2 byte pointer to asciiz hostname to resolve (can also be a dotted quad string)
diff --git a/client/ip65/function_dispatcher.s b/client/ip65/function_dispatcher.s
index 81c8821..1abf2c6 100644
--- a/client/ip65/function_dispatcher.s
+++ b/client/ip65/function_dispatcher.s
@@ -21,6 +21,7 @@
 .import ip65_error
 .import tftp_clear_callbacks
 .import tftp_download
+.import tftp_set_download_callback
 .import dns_ip
 .import dns_resolve
 .import dns_set_hostname
@@ -88,7 +89,6 @@ set_tftp_params:
   
   jsr tftp_clear_callbacks
   
-  clc
   rts
 
 nb65_dispatcher:
@@ -151,7 +151,6 @@ irq_handler_installed:
   cpy #NB65_TFTP_DOWNLOAD
   bne :+
   jsr set_tftp_params
-  bcs @tftp_error
   jsr tftp_download
   jmp @after_tftp_call
 :
@@ -292,6 +291,19 @@ irq_handler_installed:
   rts
 :  
 
+  cpy #NB65_TFTP_CALLBACK_DOWNLOAD
+  bne :+
+  jsr set_tftp_params
+  ldy #NB65_TFTP_POINTER+1
+  lda (nb65_params),y
+  tax
+  dey
+  lda (nb65_params),y  
+  jsr tftp_set_download_callback
+  jmp tftp_download
+:
+
+
   cpy #NB65_PRINT_ASCIIZ
   bne :+
   jsr print
diff --git a/client/nb65/Makefile b/client/nb65/Makefile
index fdd0233..30845f5 100644
--- a/client/nb65/Makefile
+++ b/client/nb65/Makefile
@@ -56,6 +56,7 @@ utherboot.dsk: utherboot.pg2
 
 c64boot.d64: nb65_c64_ram.prg
 	ripxplore.rb --init CbmDos $@ -a nb65_c64_ram.prg
+	ripxplore.rb $@ -a ..\test\test_cart_api.prg
 	
 $(BOOTA2.PG2): bootmenu.o $(IP65LIB) $(APPLE2PROGLIB) $(INCFILES) ../cfg/a2language_card.cfg
 	$(LD) -m bootmenu.map -C ../cfg/a2language_card.cfg -o $(BOOTA2.PG2)  $< $(IP65LIB) $(APPLE2PROGLIB)  
diff --git a/client/test/test_cart_api.s b/client/test/test_cart_api.s
index 4b9001f..8c93777 100644
--- a/client/test/test_cart_api.s
+++ b/client/test/test_cart_api.s
@@ -33,7 +33,8 @@ print_a = $ffd2
   
   .bss
   nb65_param_buffer: .res $20  
-
+  block_number: .res $0
+  
 .segment "STARTUP"    ;this is what gets put at the start of the file on the C64
 
 .word basicstub		; load address
@@ -136,7 +137,24 @@ init:
   call #NB65_PRINT_DOTTED_QUAD
   print_cr
 
-;callback test
+;tftp callback test
+  lda #0
+  sta block_number
+  lda #$FF
+  ldx #$03
+:
+  sta nb65_param_buffer,x   ;set TFTP server as broadcast address
+  dex
+  bpl :-
+  ldax #test_file
+  stax nb65_param_buffer+NB65_TFTP_FILENAME
+  ldax #tftp_callback
+  stax nb65_param_buffer+NB65_TFTP_POINTER
+  ldax #nb65_param_buffer
+  call #NB65_TFTP_CALLBACK_DOWNLOAD
+
+
+;udp callback test
   
   ldax  #64     ;listen on port 64
   stax nb65_param_buffer+NB65_UDP_LISTENER_PORT
@@ -157,6 +175,15 @@ init:
   jsr NB65_PERIODIC_PROCESSING_VECTOR
   jmp @loop_forever
   
+  
+tftp_callback:
+  inc block_number
+  print #block_no
+  lda block_number
+  jsr print_hex
+  print_cr
+  rts
+  
 udp_callback:
 
   ldax #nb65_param_buffer
@@ -260,7 +287,28 @@ get_key:
   beq get_key
   rts
   
-	.rodata
+  print_hex:
+  pha  
+  pha  
+  lsr
+  lsr
+  lsr
+  lsr
+  tax
+  lda hexdigits,x
+  jsr print_a
+  pla
+  and #$0F
+  tax
+  lda hexdigits,x
+  jsr print_a
+  pla
+  rts
+
+.rodata
+hexdigits:
+.byte "0123456789ABCDEF"
+
 test_hostname:
   .byte "RETROHACKERS.COM",0          ;this should be an A record
 
@@ -286,7 +334,10 @@ length:
   
 data:
   .byte "DATA: ",0
-  
+
+block_no:  
+  .byte "BLOCK: $",0
+
 error_code:  
   .asciiz "ERROR CODE: $"
 press_a_key_to_continue:
@@ -309,5 +360,8 @@ reply_message:
 reply_message_end:
 reply_message_length=reply_message_end-reply_message
 
+test_file:
+.byte "BOOTA2.PG2",0
+
 nb65_signature:
   .byte $4E,$42,$36,$35  ; "NB65"  - API signature
\ No newline at end of file
diff --git a/doc/nb65_api_technical_reference.doc b/doc/nb65_api_technical_reference.doc
index 695f8bf..d937267 100644
Binary files a/doc/nb65_api_technical_reference.doc and b/doc/nb65_api_technical_reference.doc differ