Memory layout of a data structure
It is often useful to control the memory layout of fields in a data structure to match an interface control definition, or to interface with hardware. Define a data structure matching the RS-232 Plug Definition. Use the 9-pin definition for brevity.
You are encouraged to solve this task according to the task description, using any language you may know.
Pin Settings for Plug (Reverse order for socket.) __________________________________________ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 _________________ 1 2 3 4 5 6 7 8 9 25 pin 9 pin 1 - PG Protective ground 2 - TD Transmitted data 3 3 - RD Received data 2 4 - RTS Request to send 7 5 - CTS Clear to send 8 6 - DSR Data set ready 6 7 - SG Signal ground 5 8 - CD Carrier detect 1 9 - + voltage (testing) 10 - - voltage (testing) 11 - 12 - SCD Secondary CD 13 - SCS Secondary CTS 14 - STD Secondary TD 15 - TC Transmit clock 16 - SRD Secondary RD 17 - RC Receiver clock 18 - 19 - SRS Secondary RTS 20 - DTR Data terminal ready 4 21 - SQD Signal quality detector 22 - RI Ring indicator 9 23 - DRS Data rate select 24 - XTC External clock 25 -
Ada
type Bit is mod 2; type Rs_232_Layout is record Carrier_Detect : Bit; Received_Data : Bit; Transmitted_Data : Bit; Data_Terminal_ready : Bit; Signal_Ground : Bit; Data_Set_Ready : Bit; Request_To_Send : Bit; Clear_To_Send : Bit; Ring_Indicator : Bit; end record; for Rs_232_Layout use record Carrier_Detect at 0 range 0..0; Received_Data at 0 range 1..1; Transmitted_Data at 0 range 2..2; Data_Terminal_Ready at 0 range 3..3; Signal_Ground at 0 range 4..4; Data_Set_Ready at 0 range 5..5; Request_To_Send at 0 range 6..6; Clear_To_Send at 0 range 7..7; Ring_Indicator at 0 range 8..8; end record;
C/C++
Note: The order of the fields is implementation-defined (i.e. the first bit might be the least-significant one or the most-significant one). On GCC and MSVC++, the first bit is the least-significant one.
struct RS232_data
{
unsigned carrier_detect : 1;
unsigned received_data : 1;
unsigned transmitted_data : 1;
unsigned data_terminal_ready : 1;
unsigned signal_ground : 1;
unsigned data_set_ready : 1;
unsigned request_to_send : 1;
unsigned clear_to_send : 1;
unsigned ring_indicator : 1;
};
The ":1" gives the number of allocated bits. For unused bits (e.g. pin 11 in the 25-pin version above) the field name can be omitted.
Forth
Low level hardware control is a typical use of Forth. None of this is standard, however, since hardware I/O mechanisms differ on different systems. Forth does not have a structure mechanism, much less bitfields. These would be represented instead via bitmask constants if doing real serial port control.
: masks ( n -- ) 0 do 1 i lshift constant loop ; 9 masks DCD RxD TxD DTR SG DSR RTS CTS RI
Example usage, assuming I/O primitives in and out:
hex
3fd constant com1-ctrl
decimal
: wait-ready
begin
com1-ctrl in
CTS and
until ;
: wait-rx
begin
com1-ctrl in
CTS and 0=
until ;
: send-byte ( b -- ) \ send assuming N81 (no parity, 8 bits data, 1 bit frame)
255 and
9 0 do
RTS com1-ctrl out
wait-ready
dup 1 and if TxD else 0 then com1-ctrl out
wait-rx
2/
loop drop ;
Of course, this is a very simplified view of the full RS-232 protocol. Also, although this represents the order of the pins in a D-9 connector, this would not necessarily be the same as the order of the bits in a control register.
OCaml
<ocaml>open ExtLib class rs232_data = object
val d = BitSet.create 9
method carrier_detect = BitSet.is_set d 0 method received_data = BitSet.is_set d 1 method transmitted_data = BitSet.is_set d 2 method data_terminal_ready = BitSet.is_set d 3 method signal_ground = BitSet.is_set d 4 method data_set_ready = BitSet.is_set d 5 method request_to_send = BitSet.is_set d 6 method clear_to_send = BitSet.is_set d 7 method ring_indicator = BitSet.is_set d 8
method set_carrier_detect b = (if b then BitSet.set else BitSet.unset) d 0 method set_received_data b = (if b then BitSet.set else BitSet.unset) d 1 method set_transmitted_data b = (if b then BitSet.set else BitSet.unset) d 2 method set_data_terminal_ready b = (if b then BitSet.set else BitSet.unset) d 3 method set_signal_ground b = (if b then BitSet.set else BitSet.unset) d 4 method set_data_set_ready b = (if b then BitSet.set else BitSet.unset) d 5 method set_request_to_send b = (if b then BitSet.set else BitSet.unset) d 6 method set_clear_to_send b = (if b then BitSet.set else BitSet.unset) d 7 method set_ring_indicator b = (if b then BitSet.set else BitSet.unset) d 8
end
- </ocaml>
Perl
use Bit::Vector::Minimal qw();
my $vec = Bit::Vector::Minimal->new(size => 24);
my %rs232 = reverse (
1 => 'PG Protective ground',
2 => 'TD Transmitted data',
3 => 'RD Received data',
4 => 'RTS Request to send',
5 => 'CTS Clear to send',
6 => 'DSR Data set ready',
7 => 'SG Signal ground',
8 => 'CD Carrier detect',
9 => '+ voltage (testing)',
10 => '- voltage (testing)',
12 => 'SCD Secondary CD',
13 => 'SCS Secondary CTS',
14 => 'STD Secondary TD',
15 => 'TC Transmit clock',
16 => 'SRD Secondary RD',
17 => 'RC Receiver clock',
19 => 'SRS Secondary RTS',
20 => 'DTR Data terminal ready',
21 => 'SQD Signal quality detector',
22 => 'RI Ring indicator',
23 => 'DRS Data rate select',
24 => 'XTC External clock',
);
$vec->set($rs232{'RD Received data'}, 1);
$vec->get($rs232{'TC Transmit clock'});
Python
This task cannot really be done efficiently in Python, as there is no type analogous to bit fields. The code below merely defines a dictionary with the pin names corresponding to entries in the dictionary.
<python># Controlling Fields in a Structure in Python
rs232 = { "PG Protective ground":1, "TD Transmitted data":2, "RD Received data":3, "RTS Request to send":4, "CTS Clear to send":5, "DSR Data set ready":6, "SG Signal ground":7, "CD Carrier detect":8, "+ voltage (testing)":9, "- voltage (testing)":10, "SCD Secondary CD":12, "SCS Secondary CTS":13, "STD Secondary TD":14, "TC Transmit clock":15, "SRD Secondary RD":16, "RC Receiver clock":17, "SRS Secondary RTS":19, "DTR Data terminal ready":20, "SQD Signal quality detector":21, "RI Ring indicator":22, "DRS Data rate select":23, "XTC External clock":24 }
rs232["RD Received data"] = 1
print rs232["TC Transmit clock"]</python>