Preparing source files for FALSIM, FALCON-A assembly language techniques

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Advanced Computer Architecture-CS501

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Advanced Computer Architecture

Lecture No. 29

Reading Material

Handouts

Slides

Summary

Introduction to FALSIM

Preparing source files for FALSIM

Using FALSIM

FALCON-A assembly language techniques

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 graphical user interface (GUI) for the FALCON-A

Assembler. This tool loads a FALCON-A assembly file with a (.asmfa) extension and

parses it. It shows the parsed 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 source code has two main modules, the 1st-pass module and

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

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

corresponds to the storage of all program variables, labels and data values in a data

structure at the implementation level. The Symbol Table 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 op-

codes, register op-codes 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 contained in the assembly file.

FALCON-A Simulator:

Figure 6 shows a snapshot of the GUI for 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.

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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 variables, addresses and labels with their respective values.

· Print Instruction Table

This feature, if selected, writes the FALCON-A instructions along with their op-codes at

the end of the list file.

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 two passes of the

assembler. The size of this window can be changed by dragging the boundary line up or

down.

Highlight: This feature 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. 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.

Memory: The memory is divided into two 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

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Instruction Register and the Program Counter while the program executes.

Highlight: The highlight option for the FALCON-A simulator is labeled as "17" in

Figure 6. This feature is similar to the way the highlight feature of the FALCON-A

Assembler works. It offers to highlight the search string which is entered as an input,

with the "All " and " Part " option. The results of the search are highlighted using the

yellow color. 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. The value in the PC must be an even address.

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

with the options of execution with/without breakpoint insertion. 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.

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

value present in a particular register.

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.

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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 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 pointers20. However, the

main program must start at address 126 or less21, 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.

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 fact that the immediate operand in the movi instruction must fit an 8-bit

field in the instruction word.

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3. Using FALSIM:

· 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. (It is

also possible to open the file by double clicking on the file name in the "Open"

window).

· 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 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

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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.

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 movi 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 temporary memory area for

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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, FFFF22

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

Figure 1

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 2

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Figure 3

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Figure 4

Figure 5

Figure 6

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Figure 7

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Figure 8

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4. FALCON-A assembly language programming techniques:

· 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

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.

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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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