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ASSEMBLY LANGUAGE
157
S123C100CE00004F5FE600C900088C001826F67F0018C018820025077C001820F59618397B
S1030000FC
S9
Table 7.3 The absolute S2-S8 machine-code file average.hex.
In the MPU world there are many different formats in common use. Although most of these de facto standards are manufacturer-specific, in the
main they can be used for any brand of MPU. The format of the machinecode file shown here is known as Motorola S1-S9. Let us look at the first
line, or record, which contains the code for the instructions in the program average.s in more detail:
Start of data record marker
Address of first data byte
Machine code
S1 23 C100 CE00004F5FE600C900088C001826F67F0018C018820025077C001820F5961839 7B
Number of bytes following marker
Checksum
The loader recognizes that a code record follows when the characters S1
are received. The characters S9 signify the end-of-file line. Code records
begin with the tally in hexadecimal of all characters after S1, followed by
the four-digit hexadecimal address of the first code byte. The core of the
record is the machine code, with typically up to 32 bytes in each line. The
final byte is known as a checksum. The checksum is calculated so that
a total count of all record bytes, excluding the record start characters,
will always give FFh (−1). This is used by the loader program to detect
download errors.
S1-S9 files are suitable for processors with addresses that can be represented as a 16-bit code (four hexadecimal digits), such as the 6800 MPU.
68000 MPUs have 24-bit Address buses, and need six hexadecimal digit
representations. 68020 and higher family members have 32-bit Address
buses, and need eight hexadecimal representations. The S2-S8 Motorola
format is similar to S1-S9 but with a six-digit address field. In a similar
manner, S3-S7 format files support processors with an eight-digit address.
An assembler will be very particular that the syntax of the source code