Q41531: How to Calculate Absolute Address; DEF SEG and PEEK Example

Article: Q41531
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BASICCOM SR# S890216-196 | mspl13_basic
Last Modified: 18-APR-1989

The DEF SEG statement in BASIC sets the current segment address for a
subsequent PEEK function, or a subsequent POKE, BLOAD, BSAVE, or CALL
ABSOLUTE statement. PEEK, POKE, BLOAD, BSAVE, and CALL ABSOLUTE can
all specify an offset (from the current segment) as an argument.

You can calculate the absolute address (the n'th byte) in memory from
the segment and offset as follows:

1. Multiply the segment address by 16 (or shift the hexadecimal
   representation 1 to the left, adding zero to the right-most
   digit; for example, &H40 times 16 equals &H400).

2. Add this value to the offset.

In the 8086 chip architecture, the addressable memory space is divided
into segments, each of which can contain up to 64K of memory. Segments
can only start on a paragraph address. A paragraph address is a byte
location that is evenly divisible by 16 bytes. Every 16th byte in
memory contains segment number n. To access specific bytes or words in
memory, you must use an offset relative to the beginning of a
specified segment.

Together, a segment and an offset provide a segmented address that can
locate any byte in the 1 megabyte of address space in the 8086
processor.

The following PEEK function returns 1 byte located at an
offset from the paragraph address of the current segment:

   DEF SEG=paddress   ' Sets paragraph address of "current" segment
   x% = PEEK(offset)  ' A byte is returned in integer variable x%

The following book contains more information about 8086-segmented
architecture and memory addressing:

   "The New Peter Norton Programmer's Guide to the IBM PC & PS/2," by
   Peter Norton and Richard Wilton (published by Microsoft Press,
   1988).

Example 1

The following two PEEK functions access the same location in memory
starting from two different segments (using decimal notation):

   DEF SEG = 0
   x% = PEEK(256)      'PEEKs at address 0000:0256 decimal
   print x%

   DEF SEG = 1         'The next segment is 16 bytes higher.
   y% = PEEK(240)      'PEEKs at address 0001:0240 decimal
   print y%

The previous lines of code print the same PEEKed value because
(0 * 16) + 256 equals (1 * 16) + 240.

Example 2

The following is another example of two PEEK functions accessing the
same location in memory starting from two different segments, this
time using hexadecimal notation:

   DEF SEG = 0
   x% = PEEK(&H417)     'PEEKs at address 0000:0417 Hexadecimal
   print x%

   DEF SEG = &H40
   y% = PEEK(&H17)     'PEEKs at address 0040:0017 Hexadecimal
   print x%

The previous lines of code accesses the same values and prints them
because they have the same absolute address: (0 hex + 417 hex) equals
(400 hex + 17 hex).

Remember, when you calculate the absolute address, you shift the
segment address 1 digit to the left in hexadecimal notation (i.e.,
multiply by 16 decimal, or 10 hex) and then add to the offset: 40 hex
times 10 hex equals 400 hex, which is added to 17 Hex.

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