Introduction to FALSIM

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Advanced Computer Architecture-CS501

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Summary

Introduction to FALSIM

Preparing source files for FALSIM

Using FALSIM

FALCON-A assembly language techniques

FALSIM

1. Introduction to FALSIM:

FALSIM is the name of the software application which consists of the

FALCON-A assembler and the FALCON-A simulator. It runs under

Windows XP.

FALCON-A Assembler:

Figure 1 shows a snapshot of the FALCON-A Assembler. This tool loads a

FALCON-A assembly file with a (.asmfa) extension and parses it. It shows

the parse results in an error log, lets the user view the assembled file's

contents in the file listing and also provides the features of printing the

machine code, an Instruction Table and a Symbol Table to a FALCON-A

listing file. It also allows the user to run the FALCON-A Simulator.

The FALCON-A Assembler has two main modules, the 1st-pass and the

2nd-pass. The 1st-pass module takes an assembly file with a (.asmfa)

extension and processes the file contents. It then creates a Symbol Table

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which corresponds to the storage of all program variables, labels and data

values in a data structure at the implementation level. If the 1st-pass

completes successfully a Symbol Table is produced as an output, which is

used by the 2nd-pass module. Failures of the 1st-pass are handled by the

assembler using its exception handling mechanism.

The 2nd-pass module sequentially processes the .asmfa file to interpret the

instruction opcodes, register opcodes and constants using the symbol table.

It then produces a list file with a .lstfa extension independent of successful

or failed pass. If the pass is successful a binary file with a .binfa extension is

produced which contains the machine code for the program in the assembly

file.

FALCON-A Simulator:

Figure 6 shows a snapshot of the FALCON-A Simulator. This tool loads a

FALCON-A binary file with a (.binfa) extension and presents its contents

into different areas of the simulator. It allows the user to execute the

program to a specific point within a time frame or just executes it, line by

line. It also allows the user to view the registers, I/O port values and memory

contents as the instructions execute.

FALSIM Features:

The FALCON-A Assembler provides its user with the following features:

Select Assembly File: Labeled as "1" in Figure 1, this feature enables the

user to choose a FALCON-A assembly file and open it for processing by the

assembler.

Assembler Options: Labeled as "2" in Figure 1.

· Print Symbol Table

This feature if selected writes the Symbol Table (produced after the

execution of the 1st-pass of the assembler) to a FALCON-A list file with an

extension of (.lstfa). The Symbol Table includes data members, data

addresses and labels with their respective values.

· Print Instruction Table

This feature if selected writes the Instruction Table to a FALCON-A list file

with an extension of (.lstfa).

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List File: Labeled as "3", in Figure 1, the List File feature gives a detailed

insight of the FALCON-A listing file, which is produced as a result of the

execution of the 1st and 2nd-pass. It shows the Program Counter value in

hexadecimal and decimal formats along with the machine code generated for

every line of assembly code. These values are printed when the 2nd-pass is

completed.

Error Log: The Error Log is labeled as "4" in Figure 1. It informs the user

about the errors and their respective details, which occurs in any of the

passes of the assembler.

Search: Search is labeled as "5" in Figure 1 and helps the user to search for

a certain input with the options of searching with "match whole" and

"match any" parts of the string. The search also has the option of checking

with/without considering "case-sensitivity". It searches the List File area

and highlights the search results using the yellow color. It also indicates the

total number of matches found.

Start Simulator: This feature is labeled as "6" in Figure 1. The FALCON-A

Simulator is run using the FALCON-A Assembler's Start Simulator option.

The FALCON-A Simulator is invoked by the user from the FALCON-A

Assembler. Its features are detailed as follows:

Load Binary File: The button labeled as "11" in Figure 6, allows the user to

choose and open a FALCON-A binary file with a (.binfa) extension. When a

file is being loaded into the simulator all the register, constants (if any) and

memory values are set.

Registers: The area labeled as "12" in Figure 6. enables, the user to see

values present in different registers before during and after execution.

Instruction: This area is labeled as "13" in Figure 6 and contains the value of

PC, address of an instruction, its representation in Assembly, the Register

Transfer Language, the op-code and the instruction type.

I/O Ports: I/O ports are labeled as "14" in Figure 6. These ports are available

for the user to enter input operation values and visualize output operation

values whenever an I/O operation takes place in the program. The input

value for an input operation is given by the user before an instruction

executes. The output values are visible in the I/O port area once the

instruction has successfully executed.

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Memory: The memory is divided into 2 areas and is labeled as "15" in

Figure 6, to facilitate the view of data stored at different memory locations

before, during and after program execution.

Processor's State: Labeled as "16" in Figure 6, this area shows the current

values of the Instruction register and the Program Counter while the program

executes.

Search: The search option for the FALCON-A simulator is labeled as "17"

in Figure 6. This feature is similar to the way the search feature of the

FALCON-A Assembler works. It offers to highlight the search string which

goes as an input, with the "All " and " Part " option. The results of the search

are highlighted in the color yellow. It also indicates the total number of

matches.

The following is a description of the options available on the button panel

labeled as "18" in Figure 6.

Single Step: "Single Step" lets the user execute the program, one instruction

at a time. The next instruction is not executed unless the user does a "single

step" again. By default, the instruction to be executed will be the one next in

the sequence. It changes if the user specifies a different PC value using the

Change PC option (explained below).

Change PC: This option lets the user change the value of PC

(Program Counter). By changing the PC the user can execute the

instruction to which the specified PC points.

Execute: By choosing this button the user is able to execute the

instructions with the options of execution with/without breakpoint

insertion (refer to Fig. 5). In case of breakpoint insertion, the user has

the option to choose from a list of valid breakpoint values. It also has

the option to set a limit on the time for execution. This "Max

Execution Time" option restricts the program execution to a time

frame specified by the user, and helps the simulator in exception

handling.

Change Register: Using the Change Register feature, the user can

change the value present in a particular register.

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Change Memory Word: This feature enables the user to change values

present at a particular memory location.

Display Memory: Display Memory shows an updated memory area,

after a particular memory location other than the pre-existing ones is

specified by the user.

Change I/O: Allows the user to give an I/O port value if the

instruction to be executed requires an I/O operation. Giving in the

input in any one of the I/O ports areas before instruction execution,

indicates that a particular I/O operation will be a part of the program

and it will have an input from some source. The value given by the

user indicates the input type and source.

Display I/O: Display I/O works in a manner similar to Display

Memory. Here the user specifies the starting index of an I/O port. This

features displays the I/O ports stating from the index specified.

2. Preparing source files for FALSIM:

In order to use the FALCON-A assembler and simulator, FALSIM,

the source file containing assembly language statements and directives

should be prepared according to the following guidelines:

· The source file should contain ASCII text only. Each line should be

terminated by a carriage return. The extension .asmfa should be used

with each file name. After assembly, a list file with the original

filename and an extension .lstfa, and a binary file with an extension

.binfa will be generated by FALSIM.

· Comments are indicated by a semicolon (;) and can be placed anywhere

in the source file. The FALSIM assembler ignores any text after the

semicolon.

· Names in the source file can be of one of the following types:

· Variables: These are defined using the .equ directive. A value must

also be assigned to variables when they are defined.

· Addresses in the "data and pointer area" within the memory: These

can be defined using the .dw or the .sw directive. The difference

between these two directives is that when .dw is used, it is not

possible to store any value in the memory. The integer after .dw

identifies the number of memory words to be reserved starting at the

current address. (The directive .db can be used to reserve bytes in

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memory.) Using the .sw directive, it is possible to store a constant or

the value of a name in the memory. It is also possible to use pointers

with this directive to specify addresses larger than 127. Data tables

and jump tables can also be set up in the memory using this directive.

· Labels: An assembly language statement can have a unique label

associated with it. Two assembly language statements cannot have the

same name. Every label should have a colon (:) after it.

Use the .org 0 directive as the first line in the program. Although the use

of this line is optional, its use will make sure that FALSIM will start

simulation by picking up the first instruction stored at address 0 of the

memory. (Address 0 is called the reset address of the processor). A jump

[first] instruction can be placed at address 0, so that control is transferred

to the first executable statement of the main program. Thus, the label

first serves as the identifier of the "entry point" in the source file. The

.org directive can also be used anywhere in the source file to force code

at a particular address in the memory.

Address 2 in the memory is reserved for the pointer to the Interrupt

Service Routine (ISR). The .sw directive can be used to store the address

of the first instruction in the ISR at this location.

Address 4 to 125 can be used for addresses of data and pointers1.

However, the main program must start at address 126 or less2, otherwise

FALSIM will generate an error at the jump [first] instruction.

The main program should be followed by any subprograms or

procedures. Each procedure should be terminated with a ret instruction.

The ISR, if any, should be placed after the procedures and should be

terminated with the iret instruction.

The last line in the source file should be the .end directive.

The .equ directive can be used anywhere in the source file to assign

values to variables.

It is the responsibility of the programmer to make sure that code does not

overwrite data when the assembly process is performed, or vice versa. As

an example, this can happen if care is not exercised during the use of the

.org directive in the source file.

3. Using FALSIM:

Any address between 4 and 14 can be used in place of the displacement field in load or

store instructions. Recall that the displacement field is just 5 bits in the instruction word.

This restriction is because of the face that the immediate operand in the movi

instruction must fit an 8-bit field in the instruction word.

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· To start FALSIM (the FALCON-A assembler and simulator), double

click on the FALSIM icon. This will display the assembler window,

as shown in the Figure 1.

· Select one or both assembler options shown on the top right corner of

the assembler window labeled as "2". If no option is selected, the

symbol table and the instruction table will not be generated in the list

(.lstfa) file.

· Click on the select assembly file button labeled as "1". This will open

the dialog box as shown in the Figure 2.

· Select the path and file containing the source program that is to be

assembled.

· Click on the open button. FALSIM will assemble the program and

generate two files with the same filename, but with different

extensions. A list file will be generated with an extension .lstfa, and a

binary (executable) file will be generated with an extension .binfa.

FALSIM will also display the list file and any error messages in two

separate panes, as shown in Figure 3.

· Double clicking on any error message highlights and displays the

corresponding erroneous line in the program listing window pane for

the user. This is shown in Figure 4. The highlight feature can also be

used to display any text string, including statements with errors in

them. If the assembler reported any errors in the source file, then these

errors should be corrected and the program should be assembled again

before simulation can be done. Additionally, if the source file had

been assembled correctly at an earlier occasion, and a correct binary

(.binfa) file exists, the simulator can be started directly without

performing the assembly process.

· To start the simulator, click on the start simulation button labeled as

"6". This will open the dialog box shown in Figure 6.

· Select the binary file to be simulated, and click open as shown in

Figure 7.

· This will open the simulation window with the executable program

loaded in it as shown in Figure 8. The details of the different panes in

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this window were given in section 1 earlier. Notice that the first

instruction at address 0 is ready for execution. All registers are

initialized to 0. The memory contains the address of the ISR (i.e., 64h

which is 100 decimal) at location 2 and the address of the printer

driver at location 4. These two addresses are determined at assembly

time in our case. In a real situation, these addresses will be

determined at execution time by the operating system, and thus the

ISR and the printer driver will be located in the memory by the

operating system (called re-locatable code). Subsequent memory

locations contain constants defined in the program.

· Click single step button labeled as "19". FALSIM will execute the

jump [main] instruction at address 0 and the PC will change to 20h

(32 decimal), which is the address of the first instruction in the main

program (i.e., the value of main).

· Although in a real situation, there will be many instructions in the

main program, those instructions are not present in the dummy calling

program. The first useful instruction is shown next. It loads the

address of the printer driver in r6 from the pointer area in the memory.

The registers r5 and r7 are also set up for passing the starting address

of the print buffer and the number of bytes to be printed. In our

dummy program, we bring these values in to these registers from the

data area in the memory, and then pass these values to the printer

driver using these two registers. Clicking on the single step button twice,

executes these two instructions.

· The execution of the call instruction simulates the event of a print

request by the user. This transfers control to the printer driver. Thus,

when the call r4, r6 instruction is single stepped, the PC changes to

32h (50 decimal) for executing the first instruction in the printer

driver.

· Double click on memory location 000A, which is being used for

holding the PB (printer busy) flag. Enter a 1 and click the change

memory button. This will store a 0001 in this location, indicating that

a previous print job is in progress. Now click single step and note that

this value is brought from memory location 000E into register r1.

Clicking single step again will cause the jnz r1, [message] instruction

to execute, and control will transfer to the message routine at address

0046h. The nop instruction is used here as a place holder.

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· Click again on the single step button. Note that when the ret r4

instruction executes, the value in r4 (i.e., 28h) is brought into the PC.

The blue highlight bar is placed on the next instruction after the call

r4, r6 instruction in the main program. In case of the dummy calling

program, this is the halt instruction.

· Double click on the value of the PC labeled as "20". This will open a

dialog box shown below. Enter a

value of the PC (i.e., 26h)

corresponding to the call r4, r6

instruction, so that it can be

executed again. A "list" of possible

PC values can also be pulled down

using, and 0026h can be selected

from there as well.

· Click single step again to enter the printer driver again.

· Change memory location 000A to a 0, and then single step the first

instruction in the printer driver. This will bring a 0 in r1, so that when

the next jnz r1, [message] instruction is executed, the branch will not

be taken and control will transfer to the next instruction after this

instruction. This is mivi r1, 1 at address 0036h.

· Continue single stepping.

· Notice that a 1 has been stored in memory location 000A, and r1

contains 11h, which is then transferred to the output port at address

3Ch (60 decimal) when the out r1, controlp instruction executes.

This can be verified by double clicking on the top left corner of the

I/O port pane, and changing the address to 3Ch. Another way to

display the value of an I/O port is to scroll the I/O window pane to

the desired position.

· Continue single stepping till the int instruction and note the changes

in different panes of the simulation window at each step.

· When the int instruction executes, the PC changes to 64h, which is the

address of the first instruction in the ISR. Clicking single step executes

this instruction, and loads the address of temp (i.e., 0010h) which is a

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temporary memory area for storing the environment. The five store

instructions in the ISR save the CPU environment (working registers)

before the ISR change them.

· Single step through the ISR while noting the effects on various registers,

memory locations, and I/O ports till the iret instruction executes. This will

pass control back to the printer driver by changing the PC to the address of

the jump [finish] instruction, which is the next instruction after the int

instruction.

· Double click on the value of the PC. Change it to point to the int

instruction and click single step to execute it again. Continue to single step

till the in r1, statusp instruction is ready for execution.

· Change the I/O port at address 3Ah (which represents the status port at

address 58) to 80 and then single step the in r1, statusp instruction. The

value in r1 should be 0080.

· Single step twice and notice that control is transferred to the movi r7,

FFFF3 instruction, which stores an error code of 1 in r1.

The instruction was originally movi r7, -1. Since it was converted to machine language

by the assembler, and then reverse assembled by the simulator, it became movi r7,

FFFF. This is because the machine code stores the number in 16-bits after sign-

extension. The result will be the same in both cases.

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Figure 1

Figure 2

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Figure 3

Figure 4

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Figure 5

Figur

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Figure 7

Figure 8

4. FALCON-A assembly language programming techniques:

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· If a signed value, x, cannot fit in 5 bits (i.e., it is outside the range -16 to

+15), FALSIM will report an error with a load r1, [x] or a store r1, [x]

instruction. To overcome this problem, use movi r2, x followed by load

r1, [r2].

· If a signed value, x, cannot fit in 8 bits (i.e., it is outside the range

128 to +127), even the previous scheme will not work. FALSIM will

report an error with the movi r2, x instruction. The following instruction

sequence should be used to overcome this limitation of the FALCON-A.

First store the 16-bit address in the memory using the .sw directive. Then

use two load instructions as shown below:

.sw x

load r2, [a]

load r1, [r2]

This is essentially a "memory-register-indirect" addressing. It has been

made possible by the .sw directive. The value of a should be less than 15.

· A similar technique can be used with immediate ALU instructions for

large values of the immediate data, and with the transfer of control (call

and jump) instructions for large values of the target address.

· Large values (16-bit values) can also be stored in registers using the mul

instruction combined with the addi instruction. The following

instructions bring a 201 in register r1.

movi r2, 10

movi r3, 20

mul r1, r2, r3

; r1 contains 200 after this instruction

addi r1, r1, 1

; r1 now contains 201

· Moving from one register to another can be done by using the instruction

addi r2, r1, 0.

· Bit setting and clearing can be done using the logical (and, or, not, etc)

instructions.

· Using shift instructions (shiftl, asr, etc.) is faster that mul and div, if the

multiplier or divisor is a power of 2.

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