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8086 REGISTERS.
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Before considering the different memory models in Turbo C, it is useful
to appreciate the function and behaviour of registers in the 8086 range
of microprocessors and to use the DOS facility DEBUG to inspect both
the registers and memory.
Intel 8088, 8086, 80286 and 80386 microprocessors have 14 Registers,
which are used to store and process numbers quickly, within the
Central Processor Unit (CPU) itself. These are 16-bit registers,
although the first four may be accessed as 8-bit parts. The word-sized
registers are as follows:
General purpose registers - AX, BX, CX and DX
Segment registers (Code, Data, Stack & Extra) - CS, DS, SS and ES
Pointer registers (Base and Stack) - BP and SP
Index registers (Source & Destination) - SI and DI
Instruction pointer register - IP | Turbo C can not access |
| these two registers. |
Status register (Flags). | |
The AX (Accumulator) register is used to store data prior to the
execution of an instruction.
The BX (Base) register is used in some memory addressing schemes. It can
be used to store the offset portion of a far pointer.
The CX (Count) register is often used as a loop counter.
The DX (Data) register is used to store 16-bit data or as an extension to
the AX register when handling 32-bit data.
The SI (Source Index) and DI (Destination Index) registers can function as
general-purpose registers and can also act as index registers. They are
also used by Turbo C for register variables.
SP (Stack Pointer) register points to the current top-of-stack and is an
offset into the stack segment.
The BP (Base Pointer) register is a secondary stack pointer, usually used
to index into the stack in order to retrieve parameters.
The base pointer (BP) register is used in C functions as a base address for
arguments and automatic variables. Parameters have positive offsets from
BP, which vary depending on the memory model and the number of registers
saved on function entry.
The segment registers are provided because of the 'segmented memory
architecture' of the 8086 microprocessor, which is designed to directly
address only 64K of memory at a time. Each 64K part of memory is known as
a segment.
The CS (Code Segment) register identifies the segment in which the program
resides.
The DS (Data Segment) register identifies the segment in which the data
(program variables) reside.
The SS (Stack Segment) register points to the stack segment and is
used in conjunction with the Stack Pointer (SP).
The ES (Extra Segment) register is used, usually, for extra data.
A simple text program, which can be created by using the Turbo C editor,
can be loaded into memory by means of the DOS debug utility by typing:
debug <filespec>
where <filespec> can be the full file specification, including drive,
directory and filename.
The screen response is the minus sign (-) and then a number of debug
commands can be used, such as d followed by the offset address to
'dump' a section of memory or r to indicate the state of the various
registers as shown below:
debug test.txt
-d 0100
4DFC:0100 46 69 72 73 74 20 6C 69-6E 65 20 6F 66 20 74 65 First line of te
4DFC:0110 78 74 0D 0A 53 65 63 6F-6E 64 20 6C 69 6E 65 20 xt..Second line
4DFC:0120 66 6F 6C 6C 6F 77 73 0D-0A 54 68 69 72 64 20 6C follows..Third l
4DFC:0130 69 6E 65 20 69 73 20 6E-65 78 74 0D 0A 46 6F 75 ine is next..Fou
4DFC:0140 72 74 68 20 6C 69 6E 65-20 69 73 20 6C 61 73 74 rth line is last
4DFC:0150 0D 0A 1A 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
4DFC:0160 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
4DFC:0170 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
-r
AX=0000 BX=0000 CX=0053 DX=0000 SP=2AB6 BP=0000 SI=0000 DI=0000
DS=4DFC ES=4DFC SS=4DFC CS=4DFC IP=0100 NV UP EI PL NZ NA PO NC
4DFC:0100 46 INC SI
-q
The final command q is used to quit debug.
It can be seen that the CS register displays the segment in which the
code resides, whilst the IP register indicates that the code starts at
the offset address 0100, which is the normal offset used by debug.
Finally, other debug commands can be used to write a new line to the
file. Enter (e) is used first to add a new line and then insert a
carriage return and line feed (OD OA). Then Name (n) is used to give
this file a new name (test2.txt) and Write (w) is employed to write
the new file, as shown below.
-e 0152 "One more line added"
-e 0165 0D 0A
-r cx
CX 0053
:67
-n test2.txt
-w
Writing 0067 bytes
This can then be checked as follows:
-d 0100
4DFC:0100 46 69 72 73 74 20 6C 69-6E 65 20 6F 66 20 74 65 First line of te
4DFC:0110 78 74 0D 0A 53 65 63 6F-6E 64 20 6C 69 6E 65 20 xt..Second line
4DFC:0120 66 6F 6C 6C 6F 77 73 0D-0A 54 68 69 72 64 20 6C follows..Third l
4DFC:0130 69 6E 65 20 69 73 20 6E-65 78 74 0D 0A 46 6F 75 ine is next..Fou
4DFC:0140 72 74 68 20 6C 69 6E 65-20 69 73 20 6C 61 73 74 rth line is last
4DFC:0150 0D 0A 4F 6E 65 20 6D 6F-72 65 20 6C 69 6E 65 20 ..One more line
4DFC:0160 61 64 64 65 64 0D 0A 00-00 00 00 00 00 00 00 00 added...........
4DFC:0170 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
-q