MultiMIPS 8051 Relocator

The Relocator is an MS Visual Basic program which relocates standard Intel format HEX

files so that they can be loaded at alternate 8051 program memory addresses.

It is not always possible to relocate a program in HEX file format to an arbitrarily chosen

load address. There are three parts of the process with which the program requires

assistance. For these parts it is helpful if the user has some of the following knowledge:

● the Intel HEX file format

● the content of the particular HEX file

difficult. The Relocator program does provide some assistance, as do the instructions and

other information in this document.

In particular, the three problem areas are:

1. MOV DPTR,#data16

2. AJMP and ACALL

3. Constants

Intel HEX File Format

The machine language is stored in ASCII code, hexadecimal representation. It can be

viewed [and even altered] with any text editor such as Windows NotePad. Each line of

the file is independent of the others. This means that the lines can be arbitrarily reordered

1

75, 31, 30, B2, 90, and 32. Lines 6 through 9 each have 16 [0x10] bytes.

Columns 4 through 7 contain the load address for the byte in column 10. The subsequent

bytes on the line, except for the check sum, are loaded into the sequentially following

addresses. For the second last line, 30 is loaded into address 0x404E, 80 is loaded into

address 0x 404F, and D2 is loaded into address 0x4050.

Each line has 00 in columns 8 and 9 except for the last line, which has 01 marking it as

end of file.

The last 2 columns of each line contain a check sum. This is the sum of all of the other

bytes on the line, ANDed with 0x00FF, and subtracted from 0x0100. For example, for the

second-last line:

possible also that a "clever" programmer is loading the two registers DPH and DPL with

data using this one instruction.

When the MultiMIPS 8051 HEX Relocator program finds MOV DPTR,#data16 in the

machine language, it asks the user to decide whether to relocate #data16 or not. The

correct procedure in each case can be determined by examining the machine language

code. More simply, if the #data16 value falls within the same range as the load addresses,

it probably needs to be relocated. This may be a trial and error situation.

AJMP and ACALL

If the program is to be relocated by a multiple of 2K bytes then AJMP and ACALL

instructions will automatically relocate with no problems. But it is not always possible to

ACALLs.

The two items of numeric information displayed are the load address for the AJMP or

ACALL instruction, and the operand address. The requirement is that these two addresses

be within the same fixed address 2K [0x800 or 2048 decimal] block of memory as is the

first byte of the instruction following the ACALL or AJMP.

The procedure to follow is:

1. Determine the load address of the ACALL or AJMP from the message displayed

by the Relocator.

2. Determine the load address of the following instruction by adding 2, using

hexadecimal arithmetic.

3. Determine the operand address from the message displayed by the Relocator.

4. Determine the address of the boundary between the load address block and the

operand address block

5. Compute the absolute difference between the boundary and the operand address.

773A : 7804

0x773A + 2 = 0x773C is the load address for the next instruction after the AJMP or

ACALL; it is in the 2K block beginning at 0x7000. 0x7804 is the operand address; it is in

the 2K block beginning at address 0x7800. The "relocate to" address must in this case be

moved back toward zero by 5 bytes so that the operand address will compute to 0x77FF,

and consequently be in the same 2K block as the load address of the instruction following

In the example given above, the constants are represented in line 1 of the file.

We must determine the location in the HEX file of the program constants so that we can

omit them from the relocation process, except that their load address must be relocated. It

is not practical for a utility program to distinguish between constants and machine

language instructions in the HEX file format. It is possible though for a person to make

this distinction, provided the person has some knowledge of the language translator

[assembler or compiler, and linker] used to prepare the HEX file, the Intel HEX file

format, and the particular HEX file itself. The Relocator displays the HEX file contents

and allows it to be sorted to assist the user with this part of the process. There are two

things the user can do.

First, locate the constants generally within the HEX file. Where the constants are located

depends on coding techniques used by the programmer, and on the language translator

used to produce the HEX file. The constants may be grouped into one place in the file,

Second, interpret the machine language as would an 8051 microcontroller. If the result

makes no sense then it may well be the program constants rather than machine language.

the operation code of a machine language instruction, and not with part of an operand.

For example, in line 3:

columns 10 and 11 contain 02, the machine language operation code for LJMP. This is a

3-byte instruction. The next 2 bytes 4000 are the operand, in this case the address to

LJMP to.

In line 5:

Columns 10&11: D2 is SETB, a 2-byte instruction with operand AB in columns 12&13.

Columns 14&15: D5 is DJNZ, a 3-byte instruction with operand 3008 in columns 16 to

Columns 35&36: 32 is RETI, a 1-byte instruction with no operands.

Columns 37&38: D3 is the check sum.

The machine language instruction 10 JBC, a 3-byte instruction with operand 9831, begins

at the end of line 6 before the checksum B0, and continues at the beginning of line 7 after

the record type 00.

If we interpret line 1 we get:

03 is RR, a 1-byte instruction

05 is INC, a 2-byte instruction with operand 00

03 is RR, a 1-byte instruction