DS80C400 Maxim, DS80C400 Datasheet - Page 75

• Manufacturer Part Number: DS80C400
• Description: The DS80C400 network microcontroller offers the highest integration available in an 8051 device
• Manufacturer: Maxim

Figure 17. User Code Signature (Required by Find-User Code)
Once a valid signature is found, the signature byte at offset 6, referred to in
examined to determine whether execution control should be transferred immediately or whether the search should
continue. If the segment address byte equals 00h or matches the most significant address byte for the 64kB block
being examined, execution is transferred to the user code. If the segment address byte does not match, that
segment address byte is used to determine the next memory block examined for a valid signature.
Exported ROM Functions
The DS80C400 ROM firmware implements many functions that are made accessible to the user application code.
For user application code to call a specific function, the location of that function must be known. The absolute
address location of each DS80C400 ROM function must be read from an export table (also found in the ROM). To
allow flexibility for future ROM firmware structural changes and improvements, the export table itself is not
connected to a specific address range, but instead a 3-Byte pointer to the start of the export table is fixed at
addresses FF0002h (XSB), FF0003h (MSB), and FF0004h (LSB). The first three bytes of the export table contain
the quantity of function entries in the export table. In 3-Byte increments, following the first three bytes, the rest of
the table contains absolute address locations for the exported ROM functions. Thus, once the export table location
has been discovered, the index for a given function/structure
(Function address = ExportTable[Index x 3]).
specific ROM function.
provided by the TCP/IP stack, socket layer, and task manager are included after the table, while the full details for
these and other exported ROM functions are covered in the High-Speed Microcontroller User’s Guide: Network
Microcontroller Supplement.
Figure 18. Finding the Location of an Exported ROM Function
FFxxxxh +(n x 3)h
80h,xxh
SJMP xx
FFxxxxh +03h
FFxxxxh +06h
FFxxxxh +09h
FFxxxxh
54h,49h,4Eh,49h
Table 18
SIGNATURE BYTES
ROM FUNCTION EXPORT TABLE
‘TINI’
Number of Functions Exported (n)
Function [index=1] Address
shows the contents of the ROM export table. Brief descriptions of the functionality
Function [2] Address
Function [3] Address
Function [n] Address
Segment Address(1)
.
.
.
Figure 18
75 of 97
illustrates the method for locating the export table and a
(Table
FF0000h
FFFFFFh
FFxxxxh
2
18) can be used to find its absolute address
USER CODE
(LOGICALLY LOCATED FF0000h–
Figure 17
FFFFFFh WHEN MROM = 1)
ROM Exported Function [3]
1
ROM Export Table
Export Table
TINI400 ROM
Address
as the segment address, is