How To Store Register In Data Memory Address

Memory Address Register

The retentivity accost annals (MAR) or address buffer besides stores the address that references memory.

From: Electronics Explained , 2010

Simple Embedded Processors

Peter Wilson , in Blueprint Recipes for FPGAs (Second Edition), 2016

8.4.4 Memory Accost Register

The retentivity address register is used to handle the address transferred to the retention unit of measurement, and this can exist handled either using a jitney approach (which we accept used in this architecture) or directly input annunciation for the retentivity. In this instance nosotros will use a jitney setting for the memory, therefore the MAR becomes a simple register which sets its output to the value of the required accost from the IR or PC when its command bespeak mar_load is loftier. The Memory Address Register (MAR) in a elementary microprocessor needs enough bits for the address. For case, if the accost requires eight bits then the The size of the register needs to be 8 bits broad.

The Retentivity Address Register (MAR) therefore has clock and reset signals, and as well the same interface to the internal processor autobus (mar_bus) defined equally a standard logic of direction inout, notwithstanding merely the offset viii bits are used.

The code for the Memory Accost Annals (MAR) is therefore given in the listing below

1 'define ADDR 8

2 'define OP 8

three

4 module mar (clk, nrst, mar_load, mar_bus, address);

half-dozen input clk;

seven input nrst;

8 input mar_load;

10 input [ 'OP + 'ADDR −one:0] mar_bus;

xi output [ 'ADDR −1:0] address;

thirteen wire [ 'OP + 'ADDR −1:0] mar_bus;

14 reg [ 'ADDR −1:0] accost;

sixteen always @ (posedge clk) begin

17 if (nrst ==0) begin

eighteen accost <= 0;

nineteen stop

twenty else begin

21 if (mar_load ==1) begin

22 address <= mar_bus [ 'ADDR −1:0];

23 end

24 terminate

25 finish

27 endmodule

We can test this past loading in a sample instruction, and then setting it valid so that it is seen on the charabanc. The test bench to achieve this is shown beneath:

1 'define OP 8

two 'define ADDR 8

v module mar_tb ();

half dozen // declare the counter signals

7 reg clk;

8 reg nrst;

nine reg mar_load;

11 reg [ 'OP + 'ADDR −1:0] data;

12 wire [ 'ADDR −1:0] accost;

xiii

14 // Set upwardly the initial variables and reset

15 initial brainstorm

xvi $display (" time ∖ t clk reset inc load valid data ");

17 $monitor (" % grand ∖ t % b % b % b % b % b ",

eighteen $ time, clk, nrst, mar_load, data, address);

19 clk = 1; // initialize the clock to 1

20 nrst = 1; // set the reset to 1 (not reset)

21 mar_load = 0;

22 data = xvi'b0000000000001111;

23 #five nrst = 0; // reset = 0 : resets the counter

24 #10 nrst = 1; // reset back to 1 : counter tin start

25 #10 mar_load = 1;

26 #10 mar_load = 0;

27 #10 data = xvi'bzzzzzzzzzzzzzzzz;

28 #200 $finish; // Finish the simulation

29 end

31 // Clock generator

32 e'er begin

33 #five clk = ˜clk; // Clock every five fourth dimension slots

34 end

36 //assign data = datareg;

38 // Connect DUT to test bench

39 mar DUT (clk,nrst,mar_load,data,accost);

41 endmodule

The resulting waveform shows the beliefs every bit predicted (Figure viii.8):

Read full chapter

URL:

https://www.sciencedirect.com/scientific discipline/article/pii/B9780080971292000088

How Microcomputers Piece of work

Louis East. FrenzelJr., in Electronics Explained (Second Edition), 2018

Control Unit

CPUs as well have several other registers, including the education register (IR); the plan counter (PC), too called the instruction counter; and the retention address annals (MAR), besides called the accost buffer.

The IR is used to store the instruction discussion. When the CPU fetches an instruction from memory, information technology is temporarily stored in the IR. The instruction is a binary give-and-take or code that defines a specific performance to be performed. The instruction give-and-take is besides chosen the op code or operation code. The CPU decodes the instruction, and so executes information technology.

The PC is really a counter and a register. Information technology stores a binary word that is used as the accost for accessing the instructions in a plan. If a programme begins with an teaching stored in retentivity location 43, the PC is kickoff loaded with the accost 43. The address in the PC is applied to the retention, causing the teaching in location 43 to be fetched and executed. After the instruction is executed, the PC is incremented (add one) to the next address in sequence, or 44. The instructions in a program are stored in sequential memory locations.

The MAR or accost buffer also stores the address that references memory. This register direct drives the address autobus and the retentivity address decoder in RAM or ROM. The MAR gets input from the PC when an instruction is to be accessed (see Fig. 6.vii). The MAR can as well be loaded with an address that is used to access data words stored in memory. To retrieve a data discussion used in an arithmetic functioning, the MAR is loaded with the binary discussion that points to the location of that word in RAM. This address is often a office of the instruction.

It is important to notation that the PC and the MAR (address buffer) have a stock-still length of so many $.25. And that limits the amount of retentiveness that tin can be accessed. For example, with a 16-bit address register, the address bus has 16 bits to accost RAM and ROM. With 16 bits, a maximum of 2¹⁶ = 65,536 words can be addressed.

There are ordinarily ii other registers, the flag and stack pointer registers. The flag or F register is an 8-flake register whose individual flip-flops are gear up and reset past the ALU as the various arithmetic and logic operations are carried out. Each flip-flop is called a flag. Equally an example, at that place are zero (Z) and carry (C) flags. If the accumulator content is nil after an operation is performed, the Z flag is set up indicating this condition. If an arithmetic operation (addition) results in a behave from the most significant bit (MSB) of the accumulator, the C flag is gear up indicating this status. These flags can exist monitored or tested past the control circuitry to change the sequence of processing.

The stack annals is a 16-scrap or larger register used to accost a selected area of RAM known equally the stack. This memory is used to shop register contents and condition information when subroutines and interrupts are used.

Read total chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128116418000060

Subprograms

Peter J. Ashenden , ... Darrell A. Teegarden , in The System Designer's Guide to VHDL-AMS, 2003

Example

Figure 9-3 shows an outline of a process taken from a behavioral model of a CPU. The process fetches instructions from memory and interprets them. Since the actions required to fetch an instruction and to fetch a data word are identical, the procedure encapsulates them in a procedure, read_memory. The procedure copies the address from the memory address register to the address autobus, sets the read signal to 'one', then activates the request bespeak. When the retention responds, the process copies the data from the data autobus signal to the memory data annals and acknowledges to the retentivity by setting the request betoken back to '0'. When the retentiveness has completed its functioning, the process returns.

The procedure is called in two places within the process. First, information technology is called to fetch an didactics. The process copies the programme counter into the memory address register and calls the process. When the procedure returns, the process copies the data from the retentiveness information register, placed there by the procedure, to the instruction register. The second call to the procedure takes place when a "load memory" pedagogy is executed. The procedure sets the memory address register using the values of the index register and some displacement, then calls the memory read procedure to perform the read functioning. When information technology returns, the process copies the data to the accumulator.

Since a procedure call is a class of sequential argument and a procedure torso implements an algorithm using sequential statements, at that place is no reason why 1 procedure cannot call some other procedure. In this case, command is passed from the calling procedure to the called procedure to execute its statements. When the chosen process returns, the calling procedure carries on executing statements until it returns to its caller.

Read full affiliate

URL:

https://www.sciencedirect.com/scientific discipline/article/pii/B9781558607491500098

Unproblematic figurer circuits

One thousand.R. Wilson , in Embedded Systems and Figurer Architecture, 2002

viii.1 G80 external connections

Let us assume that the G80 is to be made on a piece of silicon semi conductor. What signals should we bring into the exterior world, that is, what is the pin-out to be? This is mostly straightforward: clearly the contents of the memory address annals, MAR, and the G80 information bus, must be brought out to pins on the chip to allow advice with the external memory chips. Also, the control signals that indicate to the memory whether information technology is to read or write information must exist made available. In our design these are chosen RD and WR ^ane . Thus when RD is asserted, the memory volition output information stored within information technology, while when WR is asserted, the retention volition write the data on its information pins into the location indicated by the accost on its accost pins. Retrieve that past 'asserted' nosotros mean 'set to the voltage level that makes the point active'.

Nosotros also bring out a point, MREQ, that indicates that the address double-decker is carrying a new retentivity accost. We will be able to use this to warning the memory chips that a asking for their use is being made. In addition, when the G80 is powered-up, or reset, information technology must go its reset country, which sets the Program Counter to 0000 in order to start executing programme code from retention location 0000. The RESET input signal performs this office. We shall make use of these signals in the circuit diagrams of the computers that are designed in the following sections.

Read total chapter

URL:

https://www.sciencedirect.com/scientific discipline/commodity/pii/B9780750650649500092

Microprocessors

DJ Holding BSc(Eng), PhD, CEng, FIEE, MBCS, MIEEE , in Electrical Engineer'southward Reference Volume (Sixteenth Edition), 2003

15.4.4 Control and timing unit

The basic performance of a computer or microprocessor is governed by the control and timing unit (CTU) which generates the signals necessary to coordinate, synchronise and control the move and processing of all information within the arrangement. A simple external clock usually drives the unit, and this provides a time-reference bespeak from which the CTU generates the timing and command signals for the various logic subsystems in the computer. Modern high-operation processors may include a split clock management subsystem which generates multi-phase timing sequences for use by the CTU.

The control and timing unit is responsible for controlling the main operational cycle of the processor which is known every bit the 'education cycle'. The instruction cycle tin can be split into two distinct phases, the instruction fetch and the execution of the teaching. During the instruction fetch the accost of the next didactics is obtained from the programme-counter mechanism and transferred to the memory accost annals (MAR). A retentiveness reference operation is then performed on the code part or code segment of memory to read the opcode which is the first role of an didactics. The opcode data are transferred via the memory buffer register (MBR) to the educational activity register where information technology is decoded and then input to the CTU. The program-counter is then updated to betoken to the adjacent role of the education or to the next instruction.

The opcode identifies whatsoever further memory reference operations which are required to complete the instruction 'fetch'. The command unit uses the updated program counter to brand reference to successive addresses in the code function of memory to fetch whatever further parts of the educational activity, such as immediate information values or the addresses of the operands and the address of the resultand. This information is transferred to diverse temporary registers in the CPU for use during the 'execute' cycle. At the end of the instruction 'fetch', the CPU will contain all the information it requires to control the execution of the instruction and the program-counter volition be pointing to the next didactics to exist fetched (assuming that the execution cycle does not compute a new program-counter address). The various logic units used during the didactics 'fetch' bicycle are shown in Figure xv.nine in which the retentiveness and input/output discriminator $\bar{M} / IO$ is used to distinguish between memory reference operations and any operations involving peripheral systems which may use the same accost and data bus.

The opcode also defines the sequence of operations necessary to execute the instruction. During the execution part of the instruction cycle the control and timing unit will synchronise the transfer of information within the arrangement and control the functioning of the ALU. The control unit will access operand data past transferring the operand addresses from the temporary registers to the memory address register to perform memory reference operations. In practice, many processors have a complex information reference pointer which will compute the accost of the data object using non but the temporary annals but likewise base or segment registers, kickoff registers, and index registers co-ordinate to the addressing way specified in the didactics. If the estimator has a memory-to-retentiveness architecture, then operand data tin be transferred direct from firsthand admission memory to the arithmetic logic unit of measurement and resultands tin be returned direct to storage in immediate access retentivity. However, if the calculator has a register-to-register architecture, then the operand data is normally transferred to a CPU register before being processed past the arithmetic logic unit and resultand data is held in the accumulator or transferred to another CPU register. The register-to-register architecture has distinct operation advantages, specially when used with a multiple-instruction pipeline CTU, as in modern reduced instruction set (RISC) processors.

Read full affiliate

URL:

https://www.sciencedirect.com/science/article/pii/B9780750646376500150

Subprograms

Peter J. Ashenden , in The Designer's Guide to VHDL (Third Edition), 2008

vi.one Procedures

Nosotros commencement our discussion of subprograms with procedures. There are two aspects to using procedures in a model: get-go the process is declared, and so elsewhere the procedure is called. The syntax rule for a procedure declaration is

subprogram_body

procedure identifier

( parameter_interface_list )

subprogram_declarative_part

brainstorm

sequential_statement

cease [ procedure ] [ identifier ] ;

For now we will simply look at procedures without the parameter list role; we volition come back to parameters in the next department.

The identifier in a procedure declaration names the procedure. The name may be repeated at the end of the procedure declaration. The sequential statements in the body of a procedure implement the algorithm that the process is to perform and tin include any of the sequential statements that we have seen in previous chapters. A process can declare items in its declarative part for use in the statements in the procedure body. The declarations tin can include types, subtypes, constants, variables and nested subprogram declarations. The items alleged are not accessible outside of the procedure; we say they are local to the process.

Example 6.one Averaging an array of data samples

The following procedure calculates the boilerplate of a collection of information values stored in an array chosen samples and assigns the consequence to a variable called average. This procedure has a local variable full for accumulating the sum of array elements. Unlike variables in processes, procedure local variables are created anew and initialized each time the procedure is called.

procedure average_samples is

variable total : real := 0.0;

begin

affirm samples'length > 0 severity failure;

for index in samples'range loop

full := total + samples(index);

end loop;

average := total / real(samples'length);

end procedure average_samples;

The actions of a process are invoked by a procedure telephone call statement, which is yet another VHDL sequential statement. A process with no parameters is called simply past writing its proper name, as shown by the syntax dominion

procedure_call_statement

characterization :

procedure_name ;

The optional label allows united states of america to identify the procedure phone call argument. We will discuss labeled statements in Chapter 20. As an example, nosotros might include the following statement in a process:

average_samples;

The consequence of this argument is to invoke the procedure average_samples. This involves creating and initializing a new instance of the local variable full, then executing the statements in the trunk of the procedure. When the concluding statement in the procedure is completed, we say the process returns; that is, the thread of control of argument execution returns to the process from which the procedure was called, and the side by side statement in the process afterward the phone call is executed.

We can write a procedure annunciation in the declarative part of an architecture body or a process. We can likewise declare procedures within other procedures, but nosotros volition go out that until a later department. If a process is included in an architecture body'south declarative part, it tin be called from within whatever of the processes in the architecture body. On the other hand, declaring a process within a process hides it away from use by other processes.

Example vi.2 A procedure to implement behavior within a process

The outline beneath illustrates a process defined within a procedure. The procedure do_arith_op encapsulates an algorithm for arithmetic operations on two values, producing a result and a flag indicating whether the outcome is zero. It has a variable outcome, which it uses within the sequential statements that implement the algorithm. The statements likewise use the signals and other objects declared in the architecture body. The process alu invokes do_arith_op with a procedure call statement. The advantage of separating the statements for arithmetic operations into a procedure in this example is that it simplifies the body of the alu process.

compages rtl of control_processor is

type func_code is (add, decrease);

bespeak op1, op2, dest : integer;

signal Z_flag : boolean;

signal func : func_code;

…

brainstorm

alu : process is

procedure do_arith_op is

variable result : integer;

brainstorm

case func is

when add =>

result := op1 + op2;

when subtract =>

result := op1 - op2;

end case;

dest <= result after Tpd;

Z_flag <= result = 0 after Tpd;

end procedure do_arith_op;

begin

…

do_arith_op;

…

end procedure alu;

…

terminate compages rtl;

Another important use of procedures arises when some activity needs to be performed several times at dissimilar places in a model. Instead of writing several copies of the statements to perform the action, the statements tin can be encapsulated in a procedure, which is then called from each place.

Example 6.3 A retention read procedure invoked from several places in a model

The process outlined below is taken from a behavioral model of a CPU. The procedure fetches instructions from memory and interprets them. Since the actions required to fetch an educational activity and to fetch a data word are identical, the procedure encapsulates them in a procedure, read_memory . The procedure copies the address from the retention accost register to the accost bus, sets the read bespeak to 'i', then activates the asking signal. When the retentiveness responds, the procedure copies the information from the data motorbus point to the retention data register and acknowledges to the retentivity by setting the request signal back to '0'. When the memory has completed its functioning, the process returns.

instruction_interpreter : process is

variable mem_address_reg, mem_data_reg,

prog_counter, instr_reg, accumulator, index_reg : word;

…

procedure read_memory is

brainstorm

address_bus <= mem_address_reg;

mem_read <= '1';

mem_request <= 'i';

wait until mem_ready;

mem_data_reg := data_bus_in;

mem_request <= '0';

expect until not mem_ready;

cease procedure read_memory;

brainstorm

…-- initialization

loop

-- fetch adjacent didactics

mem_address_reg := prog_counter;

read_memory;-- call process

instr_reg := mem_data_reg;

…

instance opcode is

…

when load_mem =>

mem_address_reg := index_reg + deportation;

read_memory;-- telephone call process

accumulator := mem_data_reg;

…

finish case;

finish loop;

terminate process instruction_interpreter;

The process is called in two places inside the procedure. Beginning, it is chosen to fetch an instruction. The process copies the program counter into the retention address register and calls the procedure. When the procedure returns, the procedure copies the data from the memory data register, placed there past the procedure, to the pedagogy register. The second call to the procedure takes place when a "load memory" instruction is executed. The process sets the retentiveness address register using the values of the alphabetize annals and some deportation, then calls the memory read procedure to perform the read operation. When it returns, the process copies the data to the accumulator.

Since a procedure call is a grade of sequential statement and a procedure body implements an algorithm using sequential statements, there is no reason why ane procedure cannot call another procedure. In this example, control is passed from the calling procedure to the chosen procedure to execute its statements. When the chosen procedure returns, the calling procedure carries on executing statements until it returns to its caller.

Example 6.4 Nested procedure calls in a control sequencer

The process outlined below is a control sequencer for a register-transfer-level model of a CPU. It sequences the activation of control signals with a 2-phase clock on signals phase1 and phase2. The process contains two procedures, control_write_back and control_arith_op, that encapsulate parts of the command algorithm. The process calls control_arith_op when an arithmetic operation must be performed. This process sequences the control signals for the source and destination operand registers in the data path. It then calls control_write_back, which sequences the control signals for the register file in the data path, to write the value from the destination register. When this procedure is completed, it returns to the first procedure, which then returns to the process.

control_sequencer : procedure is

procedure control_write_back is

brainstorm

look until phase1;

reg_file_write_en <= '1';

expect until not phase2;

reg_file_write_en <= '0';

end procedure control_write_back;

process control_arith_op is

begin

wait until phase1;

A_reg_out_en <= 'i';

B_reg_out_en <= 'i';

wait until non phase1;

A_reg_out_en <= '0';

B_reg_out_en <= '0';

wait until phase2;

C_reg_load_en <= 'i';

wait until not phase2;

C_reg_load_en <= '0';

control_write_back;-- call procedure

end procedure control_arith_op;

…

begin

…

control_arith_op;-- telephone call procedure

…

stop procedure control_sequencer;

VHDL-87

The keyword procedure may not exist included at the end of a procedure declaration in VHDL-87. Process call statements may not exist labeled in VHDL-87.

6.1.1 Return Statement in a Process

In all of the examples higher up, the procedures completed execution of the statements in their bodies before returning. Sometimes it is useful to be able to return from the middle of a procedure, for example, as a way of handling an exceptional condition. We can exercise this using a return statement, described past the simplified syntax dominion

return_statement

characterization :

return ;

The optional label allows us to place the return statement. Nosotros will talk over labeled statements in Chapter xx. The upshot of the render statement, when executed in a process, is that the procedure is immediately terminated and control is transferred dorsum to the caller.

Example 6.5 A revised memory read procedure

The following is a revised version of the didactics interpreter process from Example vi.3. The procedure to read from memory is revised to check for the reset signal becoming active during a read operation. If it does, the procedure returns immediately, aborting the operation in progress. The process and so exits the fetch/execute loop and starts the process body again, reinitializing its state and output signals.

instruction_interpreter : process is

…

procedure read_memory is

begin

address_bus <= mem_address_reg;

mem_read <= '1';

mem_request <= '1';

await until mem_ready or reset;

if reset and then

return;

end if;

mem_data_reg := data_bus_in;

mem_request <= '0';

wait until not mem_ready;

end process read_memory;

begin

…-- initialization

loop

…

read_memory;

go out when reset;

…

end loop;

end procedure instruction_interpreter;

VHDL-87

Return statements may not be labeled in VHDL-87.

Read total chapter

URL:

https://www.sciencedirect.com/science/article/pii/B978012088785900006X