\documentclass[12pt]{article} \evensidemargin 0.50in \textwidth 5.75in \topmargin 0in \headsep .325in \textheight 8.25in \def\myfig#1#2#3{ \begin{figure}[ht] \special{isoscale #1, \the\hsize 3.0 in} \vspace{3.0in} \caption{#2} \label{#3} \end{figure} } \begin{document} \title{Introduction to Looping} \date{} \author{N. Narasimhamurthi} \maketitle \section{Objective} To become familiar with elementary loops and simple input/outputs functions. \subsection{Simple Input/Output} One of the basic functionality provided by any operating system is input/output routines. BUFFALO provides several useful functions for performing input/output. In this lab, we will look at two output functions provided by BUFFALO. Unlike in high level languages, functions in machine language are known by their addresses. The two functions we will be using are in ROM at locations {\tt \$FFB8} and {\tt \$FFBB}\footnote{We will indicate HEX values with the prefix \$. However, data that you would be entering in BUFFALO, as part of memory modify or register modify will be shown without the prefix \$ although it will be understood that the numbers are written in HEX.}. Rather than use these hard to remember and hard to recognize numbers, it is customary to give them meaningful names. Unless you have a good reason to do otherwise, it is best to use the name suggested by the vendor, in this case Motorola. The 'official' names for these functions are {\tt OUTA} and {\tt OUT1BYT} respectively. In assembly language, we make the connection between a name (technically known as a label) and a value using the EQU command as shown \begin{verbatim} OUTA EQU $FFB8 OUT1BYT EQU $FFBB \end{verbatim} NOTE: Labels should be written starting from column 1. If a line does not have a label, it should start with a space or a tab or a comment character. \subsubsection{The function {\tt OUTA}} The function {\tt OUTA} will transmit whatever is in register A over the serial communication line that is connected to the PC. What the PC does with this value depends on the terminal program that is used to communicate with the HC11. Under normal circumstances, the terminal program will interpret the value as an ASCII code and display the corresponding character on the screen. \paragraph{Exercise:} Power up HC11 and at the BUFFALO prompt try the following and write down what you see. \begin{enumerate} \item Issue the command RM (for register modify) and press the space bar till you see the {\bf A} register. Enter the value 31 and press enter as shown below: \begin{verbatim} >rm P-AAAA Y-AAAA X-AAAA A-AA B-AA C-D0 S-004A P-AAAA Y-AAAA X-AAAA A-AA 31 > \end{verbatim} Now execute the command \begin{verbatim} CALL FFB8 \end{verbatim} \item Repeat with the {\bf A} register modified with the following values: {\tt 32, 33, 34, 21, 22, 23, 24, 25, 41, 42 43 44} \end{enumerate} If you are using the simulator, turn on the log feature (by pressing both the shift keys). If you are working with a real HC11, you can copy and paste the contents of the terminal screen. \subsubsection{The function {\tt OUT1BYT}} This is a more involved output function. To start with, the value to be printed must be in memory. If the value is in a register you will have to store it in memory first. Next, the value in the {\bf X} register should be the address where the value is stored. Thus this function should be told 'where' and not 'what'. When you call this function, the function will send {\em two} characters to the PC. If the terminal program interprets these two characters as ASCII codes and displays the corresponding two characters, then the display would be the value written in HEX. {\em In addition, the function will increment the value in the {\bf X} register}. This is useful when displaying a series of memory locations. \paragraph{Exercise:} \begin{enumerate} \item Using the MM command (memory modify) enter the following values in memory locations {\tt 3000, 3001}, $\cdots$: {\tt 30 31 32 33 41 42 43 44}. Verify the values using the memory dump, MD, command. \item Using RM, the register modify command, change the value in the {\bf X} register to {\tt 3000} \item Execute the command {\tt CALL FFBB} and write down what the output was and also the value in the {\bf X} register after the command is executed. \item Repeat the the command {\tt CALL FFBB} and write down the output and the new value in the {\bf X} register. \item Repeat the last part until you have performed 7 calls to {\tt \$FFBB}. \end{enumerate} \subsection{Branching} Conditional branching in HC11 is controlled by the state of one or more hardware flags. The state of a flag depends on the most recently executed instruction that affects the flag. Thus, if your branching depends on the result of some instruction, then it is your responsibility to make sure that none of the instructions between the instruction you are interested in and the branching instruction affects the flags that control the branching instruction. Thus, it is a good idea to follow the instruction that sets the flag by the branching instruction. In this lab, we will use the following conditional branch instructions: \begin{center} \begin{tabular}{|l l|} % after \\: \hline or \cline{col1-col2} \cline{col3-col4} ... \hline BEQ & Branch if the Z flag is set \\ BNE & Branch if the Z flag is not set \\ \hline \end{tabular} \end{center} Now, the {\tt Z} flag is set after most instructions if the result of the instruction is a {\em zero}; or else it is cleared, i.e. not set. The two most important instructions that are often used to set/clear the flag are \begin{center} \begin{tabular}{|l l|} % after \\: \hline or \cline{col1-col2} \cline{col3-col4} ... \hline CMP & Perform a subtraction and discard the answer. However, set the flags. \\ TST & Same as CMP except subtract the number zero\\ \hline \end{tabular} \end{center} Thus, after the {\tt CMP} command, the {\tt Z} flag would be set if the two values that are compared are equal. Similarly, the {\tt TST} command will compare a value (memory or register) with zero and set the Z flag if the value is zero. \paragraph{Exercise:} \begin{enumerate} \item Using the HC11 reference book, identify 5 instructions that do {\em not} affect the {\bf C} flag, but affects some other flag. \item Using the HC11 reference book, can you identify any instruction that does {\em not} affect the {\bf V} flag, but affects some other flag? \item Using the HC11 reference book, identify 5 instructions that always clears the {\bf C} flag. \item Using the HC11 reference book, identify an instruction that always sets the {\bf C} flag. \end{enumerate} \subsection{Looping} \subsubsection{Counting loops} This is by far the simplest and most used loop structure. In a counting loop, we perform a specific operation a given number of times. A counter controls how many times the loop is executed. The counter could be stored in memory or kept in a register. If it is kept in a register, it is your responsibility to make sure that the register is not inadvertently changed either by your code or by some third party function that you call. If you decide to use a register, your best bet is to use either the {\bf B} register or {\bf Y} register. The structure of the loop is as follows: \begin{itemize} \item[1:] Initialize the counter to the number of times the loop is to be performed \item[2:] Perform any other initializations \item[3:] Test if the counter is zero. If so quit the loop \item[4:]Perform the desired task \item[5:] Perform any re-initializations \item[6:] Decrement the counter \item[7:] Go back to 3 \item[8:] Come here when you quit the loop \end{itemize} Note that there are two places where the code jumps to. One to (3) and the other to (8). When writing the code in assembly language, we would need two labels. In the code that follows, I have used the labels {\tt FOO} and {\tt BAR}. \paragraph{Exercises:} \begin{enumerate} \item Assemble the following code in your PC, transfer the S19 file to HC11, run the program and write down the output of the program. \begin{verbatim} ;Name: ;email: ;date: ; OUTA EQU $FFB8 ORG $2100 LDAB #$9 ; USING REGISTER B AS A COUNTER LDAA #$31 ; OTHER INITIALIZATION FOO TSTB ; SUBTRACT ZERO FROM B BEQ BAR ; QUIT IF THE Z FLAG IS SET, I.E. B=0 JSR OUTA ; DO THE TASK INCA ; RE-INITIALIZE DECB ; DECREMENT THE COUNTER BRA FOO ; GO BACK BAR SWI \end{verbatim} \item Modify the above program so that the program prints the upper case letters {\tt A} to {\tt Z}. Clearly indicate the changes you made. \item The following function is equivalent to the function shown above\footnote{You can have as many functions as you want in the same file. Make sure that when you change the ORG, the functions do not overlap. You can determine this by looking at the LST file}. \begin{verbatim} ; continued from the previous function ORG $2200 LDAB #$9 ; USING REGISTER B AS A COUNTER LDAA #$31 ; OTHER INITIALIZATION JUBJUB JSR OUTA ; DO THE TASK INCA ; RE-INITIALIZE DECB ; DECREMENT THE COUNTER BNE JUBJUB ; GO BACK SWI \end{verbatim} Verify that {\tt CALL 2100} and {\tt CALL 2200} produces the same output. Explain why this is so, and how and why the loop terminates. \end{enumerate} \subsubsection{One, two! One, two! And through and through ... Marching through memory} Often loops are combined with marching through memory and operating on consecutive memory location. In this case, the {\bf X} (and/or {\bf Y}) register is initialized to a starting memory address. Inside the loop, memory is accessed using {\tt IND,X} addressing mode. This operates on memory whose address is computed using the value in {\bf X} register. At the bottom of the loop, {\bf X} is incremented, so that next time around the loop, the operation is performed on the next memory location\footnote{In some cases the memory has to be accessed in the reverse order. In this case, {\bf X} starts at the end, and at the bottom of the loop, {\bf X} is decremented.}. The following program shows prints using {\tt OUTA} the values stored in 9 consecutive locations starting from location {\tt \$3000}. Note we are using FCB which is the assembly language equivalent of memory modify. \begin{verbatim} ORG $2300 LDAB #$09 LDX #$3000 *Don't forget the # VORPAL TSTB BEQ SWORD LDAA 0,X JSR OUTA INX DECB BRA VORPAL SWORD SWI ; FOLLOWING SAME AS MM 3000 FOLLOWED BY: 55 6F 66 4D 2D 44 62 72 6E ORG $3000 FCB $55, $6F, $66, $4D, $2D, $44, $62, $72, $6E \end{verbatim} Run the above program using {\tt CALL 2300}\footnote{Don't forget to assemble it and then transfer the S19 file to the HC11!}. Modify the above program as shown below and explain what the program does. Do you see why we do the {\tt DEX} before we access memory? \begin{verbatim} ORG $2400 LDAB #$09 LDX $3000 ABX SNICKER TSTB BEQ SNACK DEX LDAA 0,X JSR OUTA DECB BRA SNICKER SNACK SWI ORG $3000 FCB $55, $6F, $66, $4D, $2D, $44, $62, $72, $6E \end{verbatim} Now for some other useful examples. Explain what each of them does. Run the programs and using memory dump, verify that your explanation is correct. Each of the functions start with an {\tt ORG} command. \begin{verbatim} ORG $2500 LDAB #$10 LDX #$3000 CALLOOH TSTB BEQ CALLAY LDAA 0,X ADDA $10,X STAA $20,X INX DECB BRA CALLOOH CALLAY SWI ORG $3000 FCB $55, $6F, $66, $4D, $44, $62, $72, $6E FCB $41, $42, $43, $44, $45, $46, $47, $48 FCB 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16 ; NOTE: YOU CAN ENTER NUMBERS EITHER IN DECIMAL OR HEX! ; AFTER YOU RUN THE PROGRAM, DO ; MD 3000 302F ; TO SEE WHAT THE PROGRAM DOES \end{verbatim} \begin{verbatim} ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ORG $2600 LDAA #$0A JSR OUTA JSR OUTA LDAB #$10 LDX #$3020 KINGS TSTB BEQ CABBAGES JSR OUT1BYT LDAA #$2C JSR OUTA LDAA #$20 JSR OUTA DECB BRA KINGS CABBAGES LDAA #$0A JSR OUTA JSR OUTA SWI \end{verbatim} \end{document}