Model 4 ROM Explained – XDROM

This page covers the 3000H and higher portion of the Model 4 ROM which Frank Durda developed as the “XDROM” and “XROM”.

The ROM was designed to improve on certain aspects of the Model III-mode features. In physical form, it would replace either the “C” or “D” ROM.

The main features of the XDROM are:

• When running in Model III mode, the ROM driver allows the keyboard to produce all 128 ASCII character codes. The keyboard repeat rate and other parameters can also be adjusted. Key-rollover and “debouncing” are correctly handled at both 2 and 4 MHz.
• The keyboard repeat rate is adjustable.
• When running in Model III mode at 4MHZ, the real-time clock continues to keep correct time.
• The cursor blinks at the same rate regardless of the current CPU speed.
• Allows the use of a fully programmable CRT Controller (6845) in place of the preprogrammed version (68045) that came installed with the Model 4.
• All calls into the “C” ROM that are documented by LDOS and Radio Shack produce the same results as in the original part.
• The interrupt handler will process more than one event per hardware interrupt. This change makes high speed RS-232 communications far more reliable by reducing missed interrupts.
• The XDROM includes built-in tests for memory, video, floppy drive, and printer (available via a factory test option jumper.)
• Ability to boot LS-DOS 6.3 or TRSDOS 6.2 directly from a hard disk drive without using a startup diskette. (You must use the HBUILD6 utility to install a new boot track on your LS-DOS 6.3 or TRSDOS 6.2 hard disk).
• With Network III or Network 4 hardware installed, either can be booted by pressing a single key

Limits:

• Only Model 4 or 4D gate-array systems can use this ROM. It is not compatible with NGA systems.
• Only works with the latest “A” ROM (Radio Shack part number 8048364A). If you enter PRINT CHR$(PEEK(1490)) in ROM basic and get a “O”, you have the OLD ROM.
• SuperScripsit is unhappy with this ROM and requires patches.

Frank had been working on a ROM which is not entirely this XDROM. He versioned it 1(20) 10-Nov-85. He said that the features there were as follows. These MAY be in this version:

• Network 3 and Network 4 boots have been included in the USA assembly. Previously, you could have one or the other.
• Network 3 can be started by pressing “3” and Network 4 will try to start if you press “4”. Previously Network 3 (if present) had to be entered via BASIC SYSTEM command.
• Old American keyboard driver has been scrapped. Brand-new keyboard driver was written from scratch that generates all 128 ASCII characters. Model III Emulation modes are switchable and SHIFT+DOWN ARROW normally acts like the CTRL key, but this can be disabled. Keyboard has n-key rollover, limited to “squaring” in the diodeless keyboard. Repeat function continues until a new down-stroke or repeated key is released. FN keys are now repeatable also.
• Keyboard repeat rates are now adjustable.
• Boot Code will boot off floppy, one of the networks (if selected), enter ROM BASIC or boot off the hard disk drive. (The hard disk must contain magic to boot. If no magic the hard disk will not be considered a bootable device.)
• Model III mode screen/system clock runs at the correct speed regardless of CPU speed. Previously, it ran twice as fast at 4mhz mode.
• Cursor blink also remains the same regardless of CPU speed.
• If a REAL 6845 video processor is installed, it will be properly programmed for 64×16.
• All of video RAM is cleared on RESET. This eliminates the glitch-flash that is normally seen when booting Model 4 mode.
• In Image form, the 4P boot ROM [BREAK] handshake is passed to the “CROM” section instead of being ignored.
• RS232 board is now initialized to prevent it from hanging-up host computers.
• Floppy drive deselects after 3-5 seconds if no boot disk is found instead of running forever like the old version. Pressing any key restarts boot sequence.
• Hard disk code will handle the WD1000 and all versions of the WD1010-xx controller.
• Corvus (Network 4) ROM-loader has magic test for generic network and will select it over Network 4 if present.

Instructions:

Booting

The “X” and “XD” ROM can be started in five ways. If there is a floppy disk in drive 0 and no keys are pressed, the system will boot from the floppy disk drive when RESET is pressed or the power switch is turned on.

If there is no floppy disk available, the system will examine the keyboard for about three seconds. During that time, the following keys will perform these booting operations:

• If the BREAK key is pressed, the system will enter ROM BASIC as in the old ROM.
• If the 3 key is pressed, the system will attempt to boot from the Network 3 (RS-232).
• If the 4 key is pressed, the system will boot from the Network 4 if the hardware is present.
• If no key is pressed within three seconds, the ROM will check for a hard disk drive. If a drive is present, is ready, and has the correct boot track, the system will boot directly from the hard disk drive. If there is no hard disk drive, or it is not ready or does not have the correct boot track, the system will display Diskette? and will wait for a key to be pressed. When a key is pressed, the ROM will repeat all five checks. When booting off a hard disk drive, there may be a floppy in drive 0, but the door must be open.

Keyboard:

The “X”/”XD” ROM keyboard driver utilizes the CTRL key to allow the full 128 ASCII characters to be generated by the keyboard. Some codes are intercepted when using BASIC, but are available in most other programs.

The following table shows the keystrokes required to get the new characters. When possible the same keys that are used under LS-DOS/TRSDOS 6 were used.

CONTROL+, produces a [

CONTROL+. produces a ]

CONTROL+/ produces a \

CONTROL+; produces a ^

CONTROL+- produces a _

CONTROL+1 produces ASCII Code 27 (Move Cursor Up)

CONTROL+2 produces ASCII Code 28 (Move Cursor to Upper Left Corner)

CONTROL+3 produces ASCII Code 29 (Move Cursor to the Beginning of the Line)

CONTROL+4 produces ASCII Code 30

CONTROL+5 produces ASCII Code 127

CONTROL+6 produces a ~

CONTROL+7 produces a '

CONTROL+8 produces a {

CONTROL+9 produces a }

CONTROL+0 produces a |

CONTROL+@ produces ASCII CODE 0

CONTROL+: causes a screen print to be performed if there is a printer

In addition to the new keystrokes, the keyboard driver allows you to adjust how long the system will wait before starting to repeat a character. Another adjustment changes how quickly the repeated characters are generated. Other controls change the way the keyboard behaves when certain keys are pressed. All of these items are changed by modifying memory locations. This can be done from BASIC using PEEK and POKE, with DEBUG or using the MEMORY command (LDOS only).

3000 – Jump Table.

303C-306F – Embedding of (c) Text

3070 – Small routine to display the message held at (HL) and then to accept a keypress (stored in Register A)

3076H – Part of the Network 4 Controller Boot.

Jumping here as an entry is used when the FDC cannot be found. It checks to see if it is a student station (FDCless) system for education If it isn’t, it simply falls into the cassette BASIC code, which is about all they can do with the machine. This makes the assumption that if you don’t have a FDC you won’t have a hard disk you will boot a “normal” operating system off of.

This entry is used instead if Break was pressed or we never get a track 0 indication from the FDC.

Entering HERE is the CASS prompt routine, which is also a double fall through from entry at 3076 and 307C as a RAM saving measure.

309CH – BOOTSTRAP VECTOR – This routine resets all the ports, sets the interrupt to jump to 4046H, moved 76 bytes from 36AAH to 4000H, and reads disk sectors into 4000H+ until the interrupt fires. This is a VECTOR from 3015H.

By now the NMI circuitry should be shut off. Now, abort any floppy command that was executing when RESET was pressed. Do not change the drive select, as we may not have been writing on drive 0 when RESET was pressed, but we could start if we loaded the select port.

Next we program the CRTC in case there is a real one. In any event, zero the starting base registers which always works. You don’t have to do this in th 4P image as JP 0 always re-enters the real boot rom.

As a future space compression, the CRTC can be stored backwards with the trailing zeros overlaying the end of the RAM init blocks. There is no additional program cost, but you will save 4 or 5 bytes. Note: This will work for American versions, but may botch others!

30BDH – This is in the middle of the machine setup routine – While this is a pass through to set up the machine / warm boot, it is also a jump point for a failure to boot from anything. This is a VECTOR from 3012H.

At one point in time, Frank had a LD A,0CH and an OUT (FDC),A here stating that TEAC FD55’s get confused about the direction if the drive select comes too close to the restore command. So move the restore further away. Restore was only about 15-t states from the select.

That aside, at this point Frank writes that the two chunks of RAM init data have three bytes between them which are unused. The two tables must maintain their present positions or you will have to reassembly AROM and re-release Network 4. To save on code, the following LDIR moves three more bytes than it seems it should. This allows us to use the fallout HL from the last LDIR, saving three bytes in program space.

3127H – FLOPPY DRIVE – This routine will clear the stack, check the floppy drive status register after a read, and JUMP to 4300H to continue to DOS if successful, and jump to 310BH if errors were found.

313AH – HARD DRIVE – This routine is jumped from 30FDH if the last disk read did not produce any of CRC ERROR, SEEK ERROR, and NOT READY.

If we are here, then the call to 37B0H with a parameter of 20H (to do a SCAN ID) produced an error.

3180 – If we are here, then all is good to boot from the Hard Drive.

318BH – This fails out the FLOPPY DISK/HARD DRIVE routine.

3194H – NOT USED.

31A4H – DIFFERENCE BETWEEN XDROM AND XROM [Code is never used]

XDROM

XROM

31A6H – CASSETTE – Write a Timing Mark to Tape. Writes 14 .46 volt pulses to tape, 14 .00 volt pulses to tape, waits for 78H loops, and RETurns. Called from 3258H and 325FH.

31BDH – CASSETTE – This SUBROUTINE resets the cassette, sets the NMI, turns on interrupts, and RETURNs.

31C0H – CASSETTE – Turn the Cassette Motor Off. This is a VECTOR from 300CH.

31D1H – CASSETTE – This routine is to turn on the cassette – Part 1. This will remove the return address, save DE and BC, restore the return address, and than blank the **. This is a VECTOR from 300FH.

31E8H – CASSETTE – Turn On The Cassette – Actually Set the Bit Mask and Output the Command. This is only called from 31DFH (a few instructions up)

31F3H – CASSETTE – Check for a Data Error. This routine takes 60 T-States or 2 Countrs to process the data

31FFH – This puts a D on the screen over one of the *‘s to show a DATA ERROR.

3205H – CASSETTE – This routine reads 8 bits from cassette. Is not expressly jumped to, but is loaded up into a vector at 32DCH.

3214H – CASSETTE – Vector for a SLOW cassette read. Frank saved 2 bytes by putting the memory addresses into BC and then using 1 byte instructions to manipulate the memory address.

322FH – CASSETTE – Read the tape until it finds a timing mark or the BREAK is hit.

323DH – CASSETTE – Wait for the timing mark to pass and the next data to show up. Put that data into Bit 0 of D.

324EH – CASSETTE – Output to the cassette latch / clear input data latch. This routine OUTputs a 0 to the Cassette Port FFH. Note: This wasn’t here in Franks proposed revised ROM C. He had a JUMP to this location here, so he saved some bytes by relocating this routine here.

3252H – CASSETTE – Output one data byte (held in Register A) to cassette. Vector for a SLOW cassette write.

3269H – CASSETTE – Write a 0 Bit. This is accomplished by simply waiting the appropriate amount of time and doing nothing. Called only from 325DH.

326FH – CASSETTE – SLOW tape header write. TTurn on the cassette and generate a SYNC byte. his is a VECTOR from 3000H.

3285H – CASSETTE – Turn cassette on and read SYNC data. SLOW tape header read. This is a VECTOR from 3006H.

32ACH – CASSETTE – 1500 Baud Routine. Writes out a header of 256 bytes of 55H and then outputs the SYNC byte of 07FH using 8 bit bytes. FAST tape header write. This is a VECTOR from 3003H.

32C5H – CASSETTE – 1500 Baud Routine. Save all registers to the STACK, and Fill C with A (which is the byte to be written), and JUMP to cassette write.

32CBH – CASSETTE – Save all registers toe the STACK, and Fill C with A (which is the byte to be written), and GOSUB to write out the START BIT

32D2H – CASSETTE – Write to cassette WITHOUT a start bit. This routine outputs 8 bits

32DBH – CASSETTE – This routine will seek a good header, determine whether to use the rising of falling edge, and will find the sync byte. FAST tape header read. This is a VECTOR from 3009H.

3314H – CASSETTE – This will increment the FALLING EDGE count and toss us right back into the prior routine.

3317H – CASSETTE – The continues from 3312H. No idea why Frank put the prior 2 instructions right where they are – it created this extra JUMP.

3336H – CASSETTE – This is called to OUTPUT a Bit to Cassette

333FH – CASSETTE – This is the middle of the OUTPUT a Bit to Cassette Routine, except it sets DE to 2B2FH, to set the delay for a 0 Bit.

3342H – CASSETTE – This continues on, either from a JUMP in 333DH (where DE is 1217) or a pass-through (where DE is 2B2F) to continue the “write the bit” routine.

3350H – CASSETTE – This is the interruptable pulse length routine. There is no code to actually read a bit here; that is all interrupt based which is why interrupts are enabled.

3353H – 5 Instruction Loop to keep checking for BREAK

335CH – ABORT- This is the tape abort routine. It will place a BK on the screen and return to BASIC at the READY prompt.

3365H – CASSETTE – This is the cassette interrupt routine. This is a Port E0H Masked Jump. If the MASKABLE INTERRUPT is xxxxxxx1, it jumps here. Register C should be set prior to entry.

In this routine, polling takes 77 t-states for rising edge or 91 for falling edge. The CASINT routine takes 103 or 91 t-states. The total interrupt time is either 180 or 182 T-States (or 6 counts of C)

3369H – This is a jump point to use the common code at 336BH but with E=00H (for FALLING EDGE) instead of E=01H (for RISING EDGE). This is a masked interrupt jump when Port E0H is set with Bit 1 as 1 (i.e., xxxxxx1x).

336BH – Regardless of whether E is 01 or 00, continue here to process

3371H – This part of the routine is also called from 3B4CH and 3B56H.

3379H – If A matches E (the Set Level) continue here

337CH – PRINT # – This routine is to correct for PRINT # and switch to 500 baud. It checks to see if we have a PRINT # command and, if so, gets the port number, validates that the next character is a , and RETurns. This is a VECTOR from 302AH.

3388H – KEYBOARD – Frank modified the repeat keyboard routine to take into account whether we are at 2Mhz or 4Mhz and behave identically regardless. This is a VECTOR from 3024H.

In Frank’s notes to his November 1985 ROM, which may or may not match this ROM:

This keyboard driver uses algorithms to calculate character values instead of using lookup tables. It can return all 128 ASCII characters. All characters repeat, including the function keys. Function key and repeat delays are still alterable using the old RAM locations. In addition, the SHIFT+0 function and SHIFT+DOWN ARROW function can be disabled if desired. A fancy drop/assert handler has been added to handle trailing repeats which will allow very reliable keystroke pickup, but WILL show squaring very vividly if a square is generated in the keyboard. But “That’s not a bug, its a hardware problem.” Started Work 27-Jan-85 Frank Durda IV Finished (I Hope) 10-Nov-85 Frank Durda IV

3390H – This is the beginning of a loop ending in 3399H

If we are here then there was nothing different in the matrix, so test for repeats. Repeat will behave differently than the old ROM code. In particular, pressing the SHIFT key during a repeat operation will not change the character being repeated. This makes it the same as TRSDOS 6.

This is post November 1985 ROM code to adjust for 2Mhz vs 4Mhz and produce the same results regardless.

33C2H – KEYBOARD – Keyboard Repeat Routine – JUMPED from 33A4H meaning something has changed in the keyboard matrix. On entry, D is pointing to a keyboard row.

These next 2 instructions are new to the XDROM. They were NOT in Frank’s proposed revised ROM C, which started with the code at 33C6.

This next code builds a scan code from the row/column data.

Next, translate matrix bit number to binary value using 8 x row + column. This is different from what Frank had in his proposed replacement ROM C. What this used to do was add 8 * row + column, AND that against E to see if the bits match, and if a match exists exit. If no match, increment the row/column combined value, RLC, and loop back.

33F2H – KEYBOARD – If Bit 0 of Register E was a ONE at 33ECH, pick up here. At this point D has the scan code and H is 0 if the key is released, and NZ if pressed.

This code checks the key being released to see if it is the last one pressed down. If not, we can still start/continue repeating. If it is, we stop repeating.

3401H – This is a tiny routine to XOR Register A and Return

3403H – KEYBOARD – Translate keystrokes

We know we don’t have a CONTROL code, so next we test for the SHIFT key.

We know we have a SHIFTED character …

3424 is also a jump point if we have identified that a CTRL is being done.

3426H is also a jump point if NO SHIFT keys are pressed.

343FH – KEYBOARD – Check Bit 1 of Register B, and JUMP to 3477H if it is 0 and 343BH otherwise.

3445H – KEYBOARD – This routine is to handle keyboard rows 4-5 (meaning 0-9, !-), Shifted-0, * : + ; < , > . ? / = –

3467H – KEYBOARD

346BH – KEYBOARD – This routine will handle keyboard rows 4-5 but with the CTRL key down

348BH – KEYBOARD – This routine will handle keyboard row 7

349EH – KEYBOARD – Check for DOWN ARROW

This is new to XDROM. The jump here from 348D used to jump straight to the “SPCL45” routine without paying special attention to the down arrow.

34B2H – Process a RIGHT ARROW as a TAB

34BDH – KEYBOARD – Handle Row 8 (which is the F1, F2, F3, and CTRL keys)

This is new to XDROM. TThe old code was the same code, 3 times – compare to something (3C, 3D, 3EH), have HL point to something which increments by 1 each time, and JUMP.

34CBH – Network 4 BOOT – Note: Jumped here from 3503 because the key press was NOT a 3

If we are here, the UART reported back with DATA SET READY.

If we are here, the UART is ready, so we will request the boot sector and synchronize.

We have no delay here between this and the previous byte. We don’t need it we wait in the meantime for two characters to come in. Since xmit and rcv are at the same rate, we will wait long enough.

At this point we are all good to get the code.

The 256 byte code has been moved to the 4300H memory block, so now we need to release the UART and jump to the code.

34FEH – Network 4 Boot – GOSUBed by 3076H and 30E1H

This routine will check for both the ‘4’ key and the BREAK key being pressed. If the ‘4’ key is pressed, the return address that is on the stack will be popped off and processing will jump direct to BOOTNET4 code. If the ‘4’ key is not pressed and the BREAK key is, then this routine will return an NZ to the caller, otherwise a Z will be returned.

NETWORK 4 BOOT ROUTINE:
This routine will execute a boot from an external board called “Omninet”. The process requires the initialization of two new ports and the turning on of the External Bus bit in IOSEL (port 0ECH). The two values send to the two new ports (0D1H and 0D3H) are, respectively, LSB and MSB of Omninet’s address pointer. A third new port (0D0H) is the input (to the Z80A) of the data being read from the memory of the Omninet board. This port has an auto-increment function that increments the address given it in the other two ports.

3520H – This tests to see if the first byte of the RAM address to pass control to (4300H for Floppy or Hard Drive, 7300H for Network 4) is a FDH. If not, we have a problem and RETURN, with NZ set. Ohterwise, Bump the starting memory address beyond that first byte, and JUMP to it to pass control to that BOOT CODE.

This block of code is actually shared between the NETWORK 4 and the Hard Disk Boot!

3527H – DIFFERENCE BETWEEN XDROM AND XROM [Code is never used]

XDROM

XROM

3529H – MISC – REAL TIME CLOCK ROUTINE. Blink the cursor and update the clock and calendar

If we are here, then HL is greater than or equal to the year, meaning it is a new day

3593H – CLOCK – Check to see if the clock is on and exit back out if it is off OR the heartbeat shows that the clock was just updated. Pass through otherwise. Note: This routine is not expressly jumped to, but was loaded into the stack at 3529H

35A1H – CLOCK – Put the Clock 10 characters from the end of the first line. We enter this routine with HL pointing to the screen location 10 characters from the end of the first line. This is a VECTOR from 3036H.

35BCH – CLOCK – Put the DATE 8 characters from the end of the first line. We enter this routine with HL pointing to the screen location and we just jump into the prior routine with a different pointer to the DATE and a change in the delimeter to a /. This is a VECTOR from 3033H.

35C3H – INTERRUPTS – Maskable Interrupt Handler. This is a VECTOR from 3018H.

3601H – RS-232 – RS-232 Initialization. This is a VECTOR from 301BH.

362DH – RS-232 – This will zero out a bunch of RS-232 Related Ports and Memory Addresses. We wind up here if there is no RS-232 or the RS-232 CONFIGURATION CODE is 0.

3646H – RS-232 – This is the RS-232 Input Routine. This is a VECTOR from 301EH.

3661H – RS-232 – On Entry A contains a byte which was read from the RS-232 port. This routine just puts it into (HL) and RETurns.

3663H – RS-232 – RS-232 Input Routine – This checks for DATA READY and if so, takes in a character from the RS-232. Exits with NC set if DATA NOT READY.

366BH – RS-232 – This is the RS-232 Output Routine. This is a VECTOR from 3021H.

3681H – RS-232 – If the RS-232 Returned READY TO SEND, then we are here to do the send.

368CH – This the special keyboard translation for (US ONLY)

36A2H – This routine checks to see if we are 2Mhz or 4Mhz. If we are at 4Mhz then rotate the memory contents of (HL) left one bit. In either case, then RETurn to caller.

36AAH – Initial Vectors and DCBs for RAM 4000H-404FH. This is moved to RAM at 30CA

36F9H – The next 63 bytes are moved to 41E5H

While ROUTE was still present, the next 2 bytes would have been “RI” as part of the RS-232 DCB

While ROUTE was still present, the next 2 bytes would have been “RN” as part of the RS-232 Initialization DCB

3742H – OTHER – This is a vector to parse whether the current instruction on a the current line is in quotes. This is a VECTOR from 302DH.

3760 – I/O Re-Router. Based on the jump table, I suspect IX was set to 4025H

3773 – This subroutine is called by the print routine when a 01H or LINE FEED or CARRIAGE RETURN is the current character.

378AH – OTHER – This is the I/O Rerouter Routine (and when the output is screen, it is the Print Screen Routine). This is a VECTOR from 3027H.

37A5H – FLOPPY DRIVE – Called from 30A3 and 3185. Sends a RESET to the FDC, waits a DJNZ loop of 10H, and then RETurns

37A9H – This routine delays 10H and RETURNs.

37B0H – HARD DRIVE – This SUBROUTINE is called inside the HARD DRIVE routine once the READY status is set. On entry A is either 10H (Binary: 0001 0000; Bit 5 on) or 20H (Binary: 0010 0000; Bit 6 on).

37BBH – Called from 313A (beginning of the Hard Drive test). This is inside the HARD DRIVE routine once the READY status is set. If we are here, we either were called straight away at the top of the Hard Drive routine OR we got an error on the read despite the drive being READY, so we need to reset the controller.

37C5H – OTHER – This SUBROUTINE is called during WARM BOOT. It handles setting up the Port 84H Settings.

37CFH – A little routine to output A to Port 84 to reset the Model III and jump to clear the screen.

37D4H – This SUBROUTINE tests the active CPU SPEED and sets Z and Register A=0 if we are at 2Mhz and sets NZ/C and Register A=1 if we are at 4Mhz.

37DFH – RS-232 – Reads a byte from the RS-232 until a 00H is read.

37E5H – NOT USED.

31A4H – DIFFERENCE BETWEEN XDROM AND XROM [Code is never used]

These are never jumped to.

XDROM

XROM

37E8H – NOT USED.

37EAH – BASIC Command to Get Time and Date – A vector to STRING=DATE$+””+TIME$. This is a VECTOR from 3039H.

END OF REPLACEMENT ROM

What went into this:

In order to do this page I needed to use a few tools:

• TRDL, to convert a ROM/BIN file to a CMD file, as diassemblers won’t disassemble a ROM/BIN file. The command line was trld file.rom file.cmd. Lawrence Kesteloot’s TRS80-TOOL can do this as well, effective as of v2.4.1, with trs80-tool convert –start 0x1000 in.rom out.cmd.
• David Goben’s Model I/III/4 Disassembler v1.4 under Model III TRSDOS sending the output to PRINTER. It is the only disassembler which seemed to give me everything needed, including NO labels, no attempts to (mis)identify what might be text, no errors in disassembling (XOR A,A), the hex and ASCII, uniform columns so I could manipulate them easily, etc.
• TRS80GP because David Goben’s disassembler produced unusable output when I tried it with saving source to disk, but produced wonderful output when sent to the printer; and TRS80GP allows you to save the printer output!.
• Microsoft Visual Basic 6. Had to write a program to parse the columns and to add HTML cross reference jumps for jumps. I also used my prior disassemblies to populate the comment field. A waste in terms of things like PUSH HL, but a godsend for jumps to code in ROM A and B, or explaining how the C and NC jumps work.

and the following pages to help with the commenting:

• The Binary to Decimal to Hexadecimal Converter page athttps://www.mathsisnfun.com/binary-decimal-hexadecimal-converter.html.
• The ASCII CODE TABLE page at https://www.ascii-code.com/.
• The Hex Calculator Page at https://www.calculator.net/hex-calculator.htm.
• The Z-80 Opcode page at https://clrhome.org/table/.
• This sites very own RAM ADDRESS page at https://www.trs-80.com/wordpress/ram-addresses-and-routines/.
• This sites very own PORTS AND I/O DEVICES page at https://www.trs-80.com/wordpress/ports-and-i-o-devices/.
• This sites very own KEYBOARD MAP page at https://www.trs-80.com/wordpress/keyboard-map/.

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