EVBU -- BUFFALO Emulation

EVBU - An Open Source 68HC11 Simulator

Andrew Sterian
Padnos College of Engineering & Computing
Grand Valley State University

Top-Level \| BUFFALO Emulation \| Peripheral Emulation \| Waveform I/O	Version 0.6
	July 17, 2004
Service Functions \| Expressions \| Commands

BUFFALO Emulation

Several aspects of the BUFFALO monitor are emulated by EVBU, while others are not. Specifically, the following components are emulated:

The command line interface is emulated, with some differences.
The two-level interrupt vector table is emulated. Thus, installing interrupt vectors on EVBU and on a ROM-based BUFFALO system is the same (i.e., they are installed in RAM beginning at 0x00C4 rather than at high memory).
The BUFFALO service routines that do useful things (print characters, wait for keyboard input, etc.) are emulated, with some differences.
The SWI instruction terminates the program and returns to the command line prompt rather than generating a true interrupt.

The following aspects of BUFFALO are not emulated by EVBU:

There is no "ROM" in EVBU so the interrupt vectors at high memory can be overwritten, unlike a true ROM-based system.
There is no copy of BUFFALO in memory. All of the service routines are emulated by Python code.
The BUFFALO service routines execute in 0 cycles, as they are only emulated.

The sections below describe the BUFFALO emulation in greater detail.

Service Functions

The following service functions are emulated and behave very closely to their EVBU counterparts. Don't count on exact behavior, however.

rprint (0xFF82): This function prints a one-line summary of the 68HC11 registers, just as the BUFFALO RM command does.
hexbin (0xFF85): This function takes the ASCII value in register A and interprets it as a hexadecimal digit. Then, the 16-bit contents of memory locations 0x0096:0x0097 (known as SHFTREG) are shifted left by 4 bits and the hexadecimal digit interpretation of register A is stored in the lowest 4 bits.
chgbyt (0xFF8E): This function stores the contents of memory location 0x0097 (the LSB of SHFTREG) at the memory location whose address is in the X register.
upcase (0xFFA0): This function converts the ASCII character in register A to uppercase.
wchek (0xFFA3): This function sets the Z flag in the CCR if the ASCII character in register A is a blank space, comma, or tab character, otherwise the Z flag is set to 0.
input (0xFFAC): This function sets the A register to the ASCII code of a character typed at the keyboard, or 0 if no character was pressed. In the simulator, keys typed in at the command line are passed to this function immediately, without the need to press ENTER for each line (i.e., as you would expect).
outlhl (0xFFB2): This function prints the upper nibble (i.e., 4 bits) of register A as a single hexadecimal character.
outrhl (0xFFB5): This function prints the lower nibble (i.e., 4 bits) of register A as a single hexadecimal character.
outa (0xFFB8): This function prints the character whose ASCII code is in register A.
out1by (0xFFBB): This function prints the 8-bit value stored at the memory location in the X register as a two-character hexadecimal number. Then, the X register is incremented.
out1bs (0xFFBE): This function is the same as out1by (see above). A single space character is printed in addition (after the number).
out2bs (0xFFC1): This function is equivalent to calling out1by then out1bs. That is, a 16-bit value is printed followed by a space.
outcrl (0xFFC4): This function effects a carriage return.
outstr (0xFFC7): This function displays a carriage return, then calls outst0 (see below).
outst0 (0xFFCA): This function displays the ASCII string starting at the memory location stored in the X register. The string is terminated by the 8-bit value 0x04 (which is not printed). A carriage return is printed following the string. Note that EVBU limits the length of the string to 300 characters, assuming that strings this long are probably the result of forgetting the 0x04 terminating character.
inchar (0xFFCD): This function is like input (see above) except that this function does not return until a character is typed. Like input, this character is stored (i.e., its ASCII code) in the A register.
vecint (0xFFD0): This function fills the secondary RAM-based interrupt vector table (which begins at 0x00C4) with the instruction "JMP >$E371" except for interrupt vectors that have already been set to point to a different address. That is, if a JMP instruction is already found at one of the vector locations, the address is left alone.

BUFFALO Expressions

Unlike BUFFALO, constants may be entered in a variety of bases using different prefix characters. Type 'help overview' for a summary. The default number base continues to be hexadecimal. Also, when a symbol file (i.e., MAP file) is loaded, symbol names may be substituted for constants.

BUFFALO Commands

It is assumed that you are familiar with the BUFFALO command line. Below, the differences between BUFFALO commands and EVBU commands are described.

ASM

This command disassembles instructions, but does not assemble them.

BR

This new command can set execution breakpoints. Read the help description for this command (i.e., help br) for an explanation.

CYC

This new command manages the EVBU cycle counter. The number of cycles executed is displayed by simply typing cyc by itself. The cycle count can be reset to 0 by typing cyc reset. Note that the latter also affects the parallel I/O peripheral by resetting its cycle count to 0.

GO

This command works as expected but also serves to set the parallel I/O peripheral cycle count to 0. This makes the GO command different from the other commands that begin execution (like P and STOPWHEN).

L

This new command displays the source code corresponding to the current value of the program counter, if a symbol file (i.e., MAP file) has been loaded.

LOAD

The LOAD command has a different syntax than BUFFALO. Rather than 'LOAD T', the syntax is 'LOAD filename.s19' where filename.s19 is a valid S19 file.

LOADMAP

This new command manually loads a symbol file for the current program. Normally, a symbol file is sought automatically when the LOAD command is given.

MM

The MM command works as expected except for the interactive modification mode. After the current address and value are displayed, the new value to be written is typed at the input command line and the ENTER key must be pressed. This is different from the character-at-a-time mode of BUFFALO. Also, the extra control commands for going to previous bytes, etc. are not available. The interactive mode is exited by typing a single dot '.' character.

PRINT

This new command displays the value of an expression. It may be used for number conversion (for example, 'PRINT %10110011' displays the hexadecimal and decimal interpretations of the binary number 10110011. This command may also be used to display the value of a symbol if a symbol file is loaded.

PSHB

This new command pushes a single 8-bit value on the stack. This command may be useful for calling subroutines that take parameters.

PSHW

This new command pushes a single 16-bit value on the stack.

S

This new command continues program execution in the same way as a P instruction, but a temporary breakpoint is inserted at the return address. Thus, this instruction stops when the current subroutine returns (i.e., up to the next RTS instruction).

STOPAT

This new command continues program execution but a temporary breakpoint is inserted at the given address.

STOPWHEN

This new command continues program execution but stops execution after a given number of cycles have been simulated.

TN/TY

These new commands simulate a single instruction, just like T except that branches can be forced-taken (TY) or forced-not-taken (TN). That is, if the next instruction to be executed is a branch instruction, the TN command will prevent the branch from being taken, regardless of whether the branch would normally be taken. Similarly, the TY command will cause the branch to be taken, even if normally it would not be. These commands apply to the normal branch instructions as well as to the BRSET/BRCLR instructions.

VERF

This new command verifies that the contents of the specified S19 file match the contents of memory.

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