TRS-80 emulator and Z80 IDE

Code & debug — pick an example (hello world through $0033, a bouncing ball in block graphics, a random maze, the keyboard, sound through the cassette port, the clock interrupt) and press Assemble & run (Ctrl/⌘+Enter). Programs go at ORG $7000 or anywhere in RAM from $4000. With GW-80 your code is called the way the monitor’s G command calls it, and RET comes back to the > prompt. With your own ROM the page starts BASIC, answers MEMORY SIZE? (and the Model III’s Cass?) with Enter, waits for READY, then calls your code and puts every register back when it returns.

Breakpoints and stepping — click a line number to set a breakpoint, right-click it to run to it. Beside the editor are the screen, Run/Continue (F5), Pause (F6), Step (F11), Step over (F10), Step out (Shift+F11), Run to cursor (Ctrl+F10), the registers (type to change one), the flags (click to flip), the code at PC, the stack and watches. The Debugger tab has a longer disassembly with breakpoints and memory you can edit.

Files — drop a .CAS tape or a .CMD program on the screen, or use Open file…. A SYSTEM tape is loaded with GW-80’s L and started with G (the page types them); with Level II the page types SYSTEM, the file’s name and /, or CLOAD and RUN for a BASIC tape. A .CMD file’s blocks go straight into memory and it starts at its transfer address. .BAS / .TXT listings are typed into BASIC.

Type text — paste a BASIC listing or monitor commands and the page presses the keys.

Build .CMD and Build .CAS save your assembled program for other emulators or a real machine (a .CAS turns into audio with the usual tools).

Model I — the Z80 at 1.774 MHz (10.6445 MHz ÷ 6), 4–12 KB of ROM, the keyboard at $3800, 1 KB of video memory at $3C00, RAM from $4000 (16, 32 or 48 K). With the expansion interface ticked, a clock interrupt every 25 ms (40 Hz) that reading $37E0 clears. Port $FF drives the cassette (bits 0–1 out, bit 2 the motor, bit 7 in) and the 32-column wide mode (bit 3). An unmodified Model I has no bit 6 in video memory; tick the lower-case kit off to see what that does to lower case.

Model III — 2.028 MHz, 14 KB of ROM, the clock interrupt every second video field (30 times a second; 25 on 50 Hz machines), masked on port $E0 and cleared by reading $EC, and the wide mode on port $EC.

Screen — 64 × 16 characters of 6 × 12 dots; codes $80–$BF are 2 × 3 blocks (bit 0 top left … bit 5 bottom right), the 128 × 48 grid of Level II’s SET and POINT. White, green or amber phosphor (Video tab). The character shapes are my own, not Tandy’s.

Keyboard — eight rows read at $3801, $3802, $3804 … $3880; a pressed key is a 1. The page types what you type: your " key presses Shift-2, as on the TRS-80.

Sound — there’s no sound chip: programs flip the two cassette output bits, and that comes out of your speakers.

Tape — .CAS files hold the bytes the ROM reads (a leader, the $A5 sync byte, then the file). Loading is instant: the page answers the ROM’s tape routines ($0296 find the sync byte, $0235 read a byte, $0287 and $0264 write) from the file. With your own ROM you can turn that off in the Tape tab and the ROM reads 500-baud pulses from the cassette port in real time. What a program writes to tape can be downloaded as saved.cas.

No disk drives: no TRSDOS, no .DSK images. No printer, no RS-232.

The video is drawn once a frame (no Model I "snow" from video memory access).

Model III extras: the alternate character set and 1500-baud cassette in real time aren’t emulated (1500-baud tapes load through the fast-load path).

GW-80 isn’t Level II: BASIC programs and most commercial tapes need your own ROM.