Memory layout of a data structure
You are encouraged to solve this task according to the task description, using any language you may know.
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.
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
<lang 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;</lang>
ALGOL 68
<lang algol68>MODE RSTWOTHREETWO = BITS; INT
lwb rs232 = 1, carrier detect = 1, received data = 2, transmitted data = 3, data terminal ready = 4, signal ground = 5, data set ready = 6, request to send = 7, clear to send = 8, ring indicator = 9, upb rs232 = 9;
RSTWOTHREETWO rs232 bits := 2r01000000000000000000000000000000; # up to bits width, OR # rs232 bits := bits pack((FALSE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE));
print(("received data: ",received data ELEM rs232bits))</lang> Output: <lang algol68>received data: T</lang>
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. <lang c>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;
}; </lang> 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.
Since as stated before the order of bits can't be assured but it could be important if we need to interact with hardware, the best way is to define bit masks; of course actual writing/reading to/from an hardware "register" greater than a single byte must be done taking care of endianness.
D
Implementation uses tango's BitArray structure.
<lang D> module controlFieldsInStruct;
import tango.core.BitArray; import tango.io.Stdout; import tango.text.convert.Integer;
class RS232Wrapper(int Length = 9) {
static assert(Length == 9 || Length == 25, "ERROR, wrong type"); BitArray ba; static uint[char[]] _map; public:
static if (Length == 9) {
static this() {
_map = [ cast(char[])
"CD" : 1, "RD" : 2, "TD" : 3, "DTR" : 4, "SG" : 5,
"DSR" : 6, "RTS" : 7, "CTS" : 8, "RI" : 9
];
}
} else {
static this() {
_map = [ cast(char[])
"PG" : 1u, "TD" : 2, "RD" : 3, "RTS" : 4, "CTS" : 5,
"DSR" : 6, "SG" : 7, "CD" : 8, "+" : 9, "-" : 10,
"SCD" : 12, "SCS" : 13, "STD" : 14, "TC" : 15, "SRD" : 16,
"RC" : 17, "SRS" : 19, "DTR" : 20, "SQD" : 21, "RI" : 22,
"DRS" : 23, "XTC" : 24
];
}
}
this() {
ba.length = Length;
}
bool opIndex(uint pos) { return ba[pos]; }
bool opIndexAssign(bool b, uint pos) { return (ba[pos] = b); }
bool opIndex(char[] name) {
assert (name in _map, "don't know that plug: " ~ name);
return opIndex(_map[name]);
}
bool opIndexAssign(bool b, char[] name) {
assert (name in _map, "don't know that plug: " ~ name);
return opIndexAssign(b, _map[name]);
}
void opSliceAssign(bool b) { foreach (ref r; ba) r = b; }
char[] toString() {
char[] ret = "[";
foreach (name, value; _map)
ret ~= name ~ ":" ~ (ba[value]?"1":"0") ~", ";
ret ~= "]";
return ret;
}
}
int main(char[][] args) {
auto ba = new RS232Wrapper!(25);
// set all bits ba[] = 1; ba["RD"] = 0; ba[5] = 0;
Stdout (ba).newline;
return 0;
} </lang>
Output:
[RD:0, RI:1, DSR:1, SG:1, DTR:1, TC:1, TD:1, CD:1, SQD:1, +:1, -:1, SRD:1, RTS:1, SRS:1, STD:1, PG:1, SCD:1, CTS:0, DRS:1, SCS:1, XTC:1, RC:1 ]
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.
J
J does not support "structures", nor "fields in a structure". Instead, J supports arrays. And, of course, J could have labels corresponding to the elements of an array representing the state (voltage, current, logical bit value, whatever) of each pin of a 9-pin RS-232 plug: <lang j>labels=: <;._2]0 :0 CD Carrier detect RD Received data TD Transmitted data DTR Data terminal ready SG Signal ground DSR Data set ready RTS Request to send CTS Clear to send RI Ring indicator )</lang>
MATLAB
Defining structs in MATLAB is kind of bulky, making a class definition might be cleaner for this purpose. If you need to enumerate each pin rather than set the state of the pin using the name of the pin, you can use struct2cell() on the rs232 struct, which will return a cell array whose entries are the value of each of the structs fields in the order in which they were defined.
<lang MATLAB>>> rs232 = struct('carrier_detect', logical(1),... 'received_data' , logical(1), ... 'transmitted_data', logical(1),... 'data_terminal_ready', logical(1),... 'signal_ground', logical(1),... 'data_set_ready', logical(1),... 'request_to_send', logical(1),... 'clear_to_send', logical(1),... 'ring_indicator', logical(1))
rs232 =
carrier_detect: 1
received_data: 1
transmitted_data: 1
data_terminal_ready: 1
signal_ground: 1
data_set_ready: 1
request_to_send: 1
clear_to_send: 1
ring_indicator: 1
>> struct2cell(rs232)
ans =
[1] [1] [1] [1] [1] [1] [1] [1] [1]</lang>
OCaml
Library: extlib <lang 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
</lang>
Perl
<lang 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'}); </lang>
PicoLisp
PicoLisp can handle bit fields or bit structures only as bignums. They can be manipulated with '&', '|' and 'x|', or tested with 'bit?'. <lang PicoLisp># Define bit constants (for (N . Mask) '(CD RD TD DTR SG DSR RTS CTS RI)
(def Mask (>> (- 1 N) 1)) )
- Test if Clear to send
(when (bit? CTS Data)
... )</lang>
Python
The ctypes module allows for the creation of Structures that can map between the structures of C and python datatypes. Within Structures, bit fields can be created.
<lang python> from ctypes import Structure, c_int
rs232_9pin = "_0 CD RD TD DTR SG DSR RTS CTS RI".split() rs232_25pin = ( "_0 PG TD RD RTS CTS DSR SG CD pos neg" +
"_11 SCD SCS STD TC SRD RC" +
"_18 SRS DTR SQD RI DRS XTC" ).split()
class RS232_9pin(Structure):
_fields_ = [(__, c_int, 1) for __ in rs232_9pin]
class RS232_25pin(Structure):
_fields_ = [(__, c_int, 1) for __ in rs232_25pin]
</lang>
Ruby
Uses the BitStruct module, which is handy but awkward to instantiate objects. <lang ruby>require 'bit-struct'
class RS232_9 < BitStruct
unsigned :cd, 1, "Carrier detect" #1
unsigned :rd, 1, "Received data" #2
unsigned :td, 1, "Transmitted data" #3
unsigned :dtr, 1, "Data terminal ready" #4
unsigned :sg, 1, "Signal ground" #5
unsigned :dsr, 1, "Data set ready" #6
unsigned :rts, 1, "Request to send" #7
unsigned :cts, 1, "Clear to send" #8
unsigned :ri, 1, "Ring indicator" #9
def self.new_with_int(value)
data = {}
fields.each_with_index {|f, i| data[f.name] = value[i]}
new(data)
end
end
num = rand(2**9 - 1) puts "num = #{num}"
sample1 = RS232_9.new([("%09d" % num.to_s(2)).reverse].pack("B*")) puts sample1.inspect_detailed
sample2 = RS232_9.new_with_int(num) puts sample2.inspect_detailed
puts "CD is #{sample2.cd == 1 ? 'on' : 'off'}"</lang>
num = 37
RS232_9:
Carrier detect = 1
Received data = 0
Transmitted data = 1
Data terminal ready = 0
Signal ground = 0
Data set ready = 1
Request to send = 0
Clear to send = 0
Ring indicator = 0
RS232_9:
Carrier detect = 1
Received data = 0
Transmitted data = 1
Data terminal ready = 0
Signal ground = 0
Data set ready = 1
Request to send = 0
Clear to send = 0
Ring indicator = 0
CD is on
Tcl
This Tcl implementation represents the fields as bits in an integer. It provides two functions to get from symbolic pin names to the integer, and vice versa. <lang Tcl> set rs232_bits {CD RD TD DTR SG DSR RTS CTS RI}
proc rs232_encode args {
set res 0
foreach arg $args {
set pos [lsearch $::rs232_bits $arg]
if {$pos >=0} {set res [expr {$res | 1<<$pos}]}
}
return $res
} proc rs232_decode int {
set res {}
set i -1
foreach bit $::rs232_bits {
incr i
if {$int & 1<<$i} {lappend res $bit}
}
return $res
}
- ------------------------------ Test suite
foreach {test => expected} {
{rs232_encode CD} -> 1
{rs232_decode 1} -> CD
{rs232_encode CD RD TD} -> 7
{rs232_decode 7} -> {CD RD TD}
} {
catch $test res
if {$res ne $expected} {puts "$test -> $res, expected $expected"}
} </lang>