Logic Design for the Uni-bus SRC, Control Signals Generation in SRC

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Advanced Computer Architecture-CS501

Advanced Computer Architecture

Lecture No. 15

Reading Material

Vincent P. Heuring & Harry F. Jordan

Chapter 4

Computer Systems Design and Architecture

4.4

Summary

1) Logic Design for the Uni-bus SRC

2) Control Signals Generation in SRC

Logic Design for the Uni-bus SRC

In the previous sections, we have looked at both the behavioral and structural RTL for

the SRC. We saw that there is a need for some control circuitry for ensuring the proper

and synchronized functioning of the components of the data path, to enable it to carry out

the instructions that are part of the Instruction Set Architecture of the SRC. The control

unit components and related signals make up the control path. In this section, we will talk

about

· Identifying the control signals required

· The external CPU interface

· Memory Address Register (MAR), and Memory Buffer Register (MBR) circuitry

· Register Connections

We will also take a look at how sign extension is performed. This study will help us

understand how the entire framework works together to ensure that the operations of a

simple computer like the SRC are carried out in a smooth and consistent fashion.

Identifying control signals

For any of the instructions that are a part of the instruction set of the SRC, there are

certain control signals required; these control signals may be to select the appropriate

function for the ALU to be performed, to select the appropriate registers, or the

appropriate memory location.

Any instruction that is to be executed is first fetched into the CPU. We look at the control

signals that are required for the fetch operation.

Control signals for the fetch operation

Table 1 lists the control signals that are needed to ensure the synchronized register

transfers in the instruction fetch phase. Note that we use uppercase for control signals as

we have been using lowercase for the instruction mnemonics, and we want to distinguish

between the two. Also note that control signals during each time slot are activated

simultaneously, and that the control signals for successive time slots are activated in

sequence. If a particular control signal is not shown, its value is zero.

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As shown in the Table: 1, some control signals are to let register values to be written onto

buses, or read from the buses. Similarly, some signals are required to read/ write memory

contents onto the bus. The memory is assumed to be fast enough to respond during a

given time slot; if that is not true, wait states have to be inserted. We require four control

signals to be issued in the time step T0:

PCout: This control signal allows the contents of the Program Counter register to be

written onto the internal processor bus.

LMAR: This signal enables write onto the memory address register (MAR), thus the

value of PC that is on the bus, is copied into this register

INC4: It lets the PC value to be incremented by 4 in the ALSU, and result to be

stored in C. Notice that the value of PC has been received by the ALSU as an

operand. This control signal allows the constant 4 to be added to it.

The ALSU is assumed to include an INC4 function

LC: This enables the input to the register C for writing the incremented value of PC

onto it.

During the time step T1, the following control signals are applied:

LMBR: This enables the "write" for the register MBR. When this signal is activated,

whatever value is on the bus, can be written into the MBR.

MRead: Allow memory word to be gated from the external CPU data bus into the

MBR.

MARout: This signal enables the tri-state buffers at the output of MAR.

Cout: This will enable writing of the contents of register C onto the processor's

internal data bus.

LPC: This will enable the input to the PC for receiving a value that is currently on the

internal processor bus. Thus the PC will receive an incremented value.

At the final time step, T2, of the instruction fetch phase, the following control signals

are issued:

MBRout: To enable the tri-state buffers with the MBR.

LIR: To allow the IR read the value from the internal bus. Thus the instruction stored

in the MBR is read into the Instruction Register (IR).

Uni-bus SRC implementation

The uni-bus implementation of the SRC data path is given in the Fig.1. We can now

visualize how the control signals in mutually exclusive time steps will allow the

coordinated working of instruction fetch cycle.

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Similar control signals will allow the instruction execution as well. We have already

mentioned the external CPU buses that read from the memory and write back to it. In the

given figure, we had not shown these external (address and data buses) in detail. Fig.2

will help us understand this external interface.

External CPU bus activity

Let us take up a sample problem to further enhance our understanding of the external

CPU interface. As mentioned earlier, this interface consists of the data bus/ address bus,

and control signals for enabling memory read and write.

Example problem:

(a) What will be the logic levels on the external SRC buses when each of the given SRC

instruction is executing on the processor? Complete Table: 2. all numbers are in the

decimal number system, unless noted otherwise.

(b) Specify memory addressing modes for each of the SRC instructions given in Table: 2.

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Assumptions:

· All memory content is aligned properly.

In other words, all the memory accesses start at addresses divisible by 4.

Value in the PC = 000DC348h

Memory map with assumed values

Register map with assumed values

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Solution Part (a):

(Note that the SRC uses the big-endian storage format).

Solution part (b):

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Notes:

Relative addressing is always PC relative in the SRC

***

Displacement addressing mode is the same as Based or Indexed in the SRC. It is

also the same as Register Relative addressing mode

Memory address register circuitry

We have already talked about the functionality of the MAR. It provides a temporary

storage for the address of memory location to be accessed. We now take a detailed look

at how it is interconnected with other components. The MAR is connected directly to the

CPU internal bus, from which it is loaded (receives a value). The LMAR signal causes

the contents of the internal CPU bus to be loaded into the MAR. It writes onto the CPU

external address bus. The MARout signal causes the contents of the MAR to be placed on

the address bus. Thus, it provides the addresses for the memory and I/O devices over the

CPU's address bus. A set of tri-state buffers is provided with these connections; the tri-

state buffers are controlled by the control signals, which in turn are issued when the

corresponding instruction is decoded. The whole circuitry is shown in Fig.6.

Memory buffer register circuitry

The Memory Buffer Register (MBR) holds the value read from the memory or I/O

device. It is possible to load the MBR from the internal CPU bus or from the external

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CPU data bus. The MBR also drives the internal CPU bus as well as the external CPU

data bus. Similar to the MAR register, tri-state buffers are provided at the connection

points of the MBR, as illustrated in the Fig.7.

The register file containing the General Purpose Registers is programmer visible.

Instructions may refer to any of these registers, as source operands in an operation or as

the destination registers. Appropriate circuitry is needed to enable the specified register

for read/ write. Intuitively, we can tell that we require connections of the register to the

CPU internal bus, and we need control signals that will enable specified registers to be

read/ write enabled as a corresponding instruction is decoded. Fig.8 illustrates the register

connections and the control signals generation in the uni-bus data path of the SRC. We

can see from this figure that the ra, rb and rc fields of the Instruction Register specify the

destination and source registers. The control signals RAE, RBE and RCE can be applied

to select any of the ra, rb or rc field respectively to apply its contents to the input of 5-to-

32 decoder. Through the decoder, we get the signal for the specific register to be

accessed. The BUS2R control signal is activated if it is desired to write into the register.

On the other hand, if the register contents are to be written to the bus, the control signal

R2BUS is activated.

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Alternate control circuitry for register selection

Fig.9 illustrates an alternate circuitry that implements the register connections with the

internal processor bus, the instruction register fields, and the control signals required to

coordinate the appropriate read/write for these registers. Note that this implementation is

somewhat similar to our earlier implementation with a few differences. It illustrates the

fact that the implementations we have presented are not necessarily the only solutions,

and that there may be other possibilities.

In this alternate circuitry, there is a separate 5-to-32 decoder for each of the register fields

of the instruction register. The output of these decoders is allowed to be read out and

enables the decoded register, if the control signal (RAE, RBE or RCE) is active.

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Control signals Generation in SRC

We take a few example instructions to study the control signals that are required in the

instruction execution phase.

Control signals for the add instruction

The add instruction has the following syntax:

add ra, rb, rc

Table: 4 lists the control signals that are applied at each of the time steps. The first three

steps are of the instruction fetch phase, and we have already discussed the control signals

applied at this phase.

Table: 4

At time step T3, the control RBE is applied, which will enable the register rb to write its

contents onto the internal CPU bus, as it is decoded. The writing from the register onto

the bus is enabled by the control signal R2BUS. Control signal LA allows the bus

contents to be transferred to the register A (which will supply it to the ALSU). At time

step T4, the control signals applied are RCE, R2BUS, ADD, LC, to respectively enable

the register rc, enable the register to write onto the internal CPU bus (which will supply

the second operand to the ALSU from the bus), select the add function of the ALSU

(which will add the values) and enable register C (so the result of the addition operation

is stored in the register C). Similarly in T5, signals Cout, RAE and BUS2R are activated.

Sign extension

When we copy constant values to registers that are 32 bits wide, we need to sign extend

the values first. These values are in the 2's complement form, and to sign-extend these

values, we need to copy the most significant bit to all the additional bits in the register.

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We consider the field c2, which is a 17 bit constant. Sign extension of c2 requires that we

copy c2<16> to all the left-most bits of the destination register, in addition to copying the

original constant values to the register. This means that bus<31...17> should be the same

as c2<16>. A 15 line tri-state buffer can perform this sign extension. So we apply c2<16>

to all the inputs of this tri-state buffer as illustrated in the Fig.10.

Structural RTL for the addi instruction

We now return to our study of the control signals required in the instruction execute

phase. We have already looked at the add instruction and the corresponding signals. Now

we take a look at the addi (add immediate) instruction, which has the following syntax:

addi ra, rb, c2

Table: 5 lists the RTL and the control signals for the addi instruction:

The table shows that the control signals for the addi instruction are the same as the add

instruction, except in the time step T4. At this time step, the control signals that are

applied are c2out, ADD and LC, to respectively do the following:

Enable the read of the constant c2 (which is sign extended) onto the internal processor

bus. Add the values using the ALSU and finally assign the result to register C by

enabling write for this register.

To place a 0 on the bus

When the field rb is zero, for instance, in the load and store instructions, we need to

place a zero on the bus. The given circuit in Fig.11 can be used to do this.

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Note that, by default, the value of register R0 is 0 in some cases. So, when the selected

is not enabled, and instead a hardwired 0 is output from the tri-state buffer onto the CPU

internal bus. An alternate circuitry for achieving the same is shown in the Fig.12.

Control signals for the ld instruction

Now we take a look at the control signals for the load instruction. The syntax of the

instruction is:

ld ra, c2 (rb)

Table: 6 outlines the control signals as well as the RTL for the load instruction in the

SRC.

The first three steps are of the instruction fetch phase. Next, the control signals issued

are:

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RBE is issued to allow the register rb value to be read

R2BUS to allow the bus to read from the selected register

LA to allow write onto the register A. This will allow the CPU bus contents to be written

to the register A.

At step T4 the control signals are:

c2out to allow the sign extended value of field c2 to be written to the internal CPU bus

ADD to instruct the ALSU to perform the add function.

LC to let the result of the ALSU function be stored in register C by enabling write of

Control signals issued at step T5:

Cout is to read the register C, this copies the value in C to the internal CPU bus.

LMAR to enable write of the Memory Address Register (which will copy the value

present on the bus to MAR). This is the effective address of memory location that is to be

accessed to read (load) the memory word.

During the time step T6:

MARout to read onto the external CPU bus (the address bus, to be more specific), the

value stored in the MAR. This value is an index to memory location that is to be

accessed.

MRead to enable memory read at the specified location, this loads the memory word at

the specified location onto the CPU external data bus.

LMBR is the control signal to enable write of the MBR (Memory Buffer Register). It

will obtain its value from the CPU external data bus.

Finally, the control signals issued at the time step T7 are:

MBRout is the control signal to allow the contents of the MBR to be read out onto the

CPU internal bus.

RAE is the control signal for the destination register field ra. It will let the actual index of

the ra register be encoded, and

BUS2R will let the appropriate destination register be written to with the value on the

CPU internal bus.

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