TRS-80 Model 4

The computer has the same all-in-one cabinet as the Model III, adopting a more contemporary-looking beige color scheme instead of the black and gray used on the Models I/III. The Model 4's case also switched from spray-painted translucent plastic to molded plastic, ensuring that the coloring is not vulnerable to peeling or rubbing.

The Model 4 uses WD1770/1773 floppy controllers instead of the WD1791, which allows for a larger gap between the index hole and first sector; later releases of TRSDOS and LDOS were modified for compatibility with the controller.

The Model 4 shipped with TRSDOS 6, identical to Logical Systems's LDOS 6.00 third-party operating system (itself an enhancement to older versions of TRSDOS). When the Model 4 boots into TRSDOS 6, the video display switches into 80×24 mode and the entire 64KB address space is mapped as RAM.

When booted from a Model III operating system disk, the Model 4 enters 64×16 video mode and maps Model III ROMs from address zero, making it capable of running Model III software. Model 4 features can be accessed in Model III mode by manipulating the computer's hardware ports. The LDOS 5.3 update supports SYSTEM (FAST) to enable the Model 4's 4 MHz clock speed. Misosys Inc. sold a Model 4 Hardware Interface Kit which enables the extra keys on the Model 4 keyboard, and in a 128 KB Model 4, the banked memory.[9] Another company called Intellitech sold a program called Supermod4 that allows Model III programs running on a Model 4 to activate the 4 megahertz CPU clock, larger video display, the speaker and the function keys.[10] In August 1985 80 Micro magazine published a DoubleDuty-like task switching program that activates the external RAM banks on a 128 KB Model 4 from within Model III mode.[11]

The Model 4 can run CP/M without modification, unlike the Model I and III.[6][12] Digital Research produced a version of CP/M 3.0 for the Model 4,[13] but it is buggy and actually provides a smaller Transient Program Area than the non-banked CP/M 2.2.[note 1] Montezuma Micro sold a version of CP/M 2.2 that was customized for the Model 4's hardware: banked RAM, reverse video and assignable codes for the function keys.[14] It has a utility for reading and writing CP/M disk formats of many other brands of computer. Montezuma sold a terminate-stay-resident program they called Monte's Window, which provided functionality similar to Borland Sidekick. Its code resided entirely in the banked RAM of a 128K Model 4; no user memory was occupied.[15]

One notable program available only for the Model 4 was marketed by Radio Shack as DoubleDuty.[16][17][18] This is one of the first task-switching programs available for any microcomputer. It uses the upper 64KB of a 128KB machine to keep resident a second TRSDOS application, which can be switched instantly with another application loaded into the main 64KB. A third partition is available for TRSDOS library commands, such as DIR. DoubleDuty first appeared in Radio Shack's 1985 Computer Catalog (RSC-12), the same year that IBM's Topview, Apple's Switcher, and Quarterdeck's DESQview first became available. DoubleDuty was written by Randy Cook, the author of the first version of TRSDOS for the original Model I.

The Model 4's memory mapping and OS more closely resemble the TRS-80 Model II than it does the Model III. Like the Model II, there is no ROM-based OS and all OS software was RAM-resident and loaded from disk at bootup. There is only a small boot loader ROM which checks for the presence of a Model 4 OS disk and if one is not detected, it defaults to Model III mode. Also like the Model II, the OS uses vectored API calls instead of absolute addresses, and software developers were encouraged to use the API calls rather than low-level hardware access to ensure compatibility with future iterations of TRSDOS 6.[note 2]

Rather than accessing the memory-mapped keyboard and video directly as on the Models I and III, TRSDOS SVCs are used. This method permits I/O redirection over all the computer's devices, including the disk drives. This makes possible, for example, to "print" a document to a disk file so it can be printed at a later time. Another frequent use of I/O redirection is redirecting video output to the printer for permanent hardcopy of a program run. TRSDOS supplies the LINK, ROUTE, and FILTER commands to enable these capabilities. This method also ensures consistent communication between memory resident modules attached to the OS's logical devices. Some applications programmers, however, circumvent this device-independent approach by physically accessing the computer hardware, such as Radio Shack's SuperScripsit word processor; its programmers insisted on having the CTL255 routine built into the keyboard driver expressly for this purpose.[19] Another offender was Anitek Software's Lescript. These two applications ignore any filter programs attached to the keyboard device, depriving the user of some terminate and stay resident programs, such as Misosys' Sidekick-like Pro-WAM[20] Window Application Manager, as well as the TRSDOS KSM/FLT keystroke multiplier macro facility.

The disk drives are identical to the Model III, consisting of one or two single-sided full-height 5.25" drives (various brands of disk drive were used in the Model 4, while the Model III had exclusively Tandons), providing 180KB of storage with TRSDOS. An additional two drives can connect via the external floppy port. Any floppy drive with the Shugart-style 34-pin interface can be used; thus it is possible to upgrade a Model 4 to use double-sided, 80 track, or even 3.5" 720 KB (low density) floppies. Drive select is implemented via the card edge connectors for the drives; the connector for each drive bay has certain conductors missing that identify the lower drive as system drive zero, and the upper drive as system drive one. Thus the drives do not have drive select jumpers, and are identical. This makes the drives in a dual drive Model 4 interchangeable. Replacement with third-party double-sided drives requires use of a new drive cable and connectors with all conductors present; in this case drive select is made with jumpers on the drives.

External hard disks were available using the computer's 50 pin expansion card-edge, which also permit other external hardware requiring direct access to the Z80 buses. These include Atari style joystick adapters or the line of data acquisition devices marketed by Alpha Products. A parallel printer can be added using another card-edge connector provided for this purpose. An RS-232 serial port was optional on the original versions of the Model 4.

The baseline Model 4 uses a 16kx1 DRAM known as the 8040517. This chip, also used in 16k TRS-80 CoCo 2s, is functionally identical to a 4116 DRAM, but only uses +5V power instead of the 4116's tri-voltage power. 64k and 128k models use standard 4164 DRAMs. Unlike previous machines, the RS-232 port is standard equipment rather than an extra cost option. The RS-232 and printer ports are also moved to the back instead of the underside of the computer, making them more easily accessible.

The Model 4 was announced in the same press release (April 1983) as was the TRS-80 Model 100 laptop. The two computers were often marketed by Tandy/Radio Shack as a complementary pair. Model 100 cassette tapes are readable on the Model 4 with its cassette interface and a TRSDOS 6 utility called TAPE100/CMD, which store Model 100 programs and data as TRSDOS disk files. Programs and files can be transferred back and forth via an RS-232 serial null modem cable, or over a telephone line using a modem on the Model 4 end (the Model 100 has a built-in modem). Both the Model 4 and the Model 100 came with terminal software as a built-in feature.

Early versions of the Model 4 mainboard were designed to accept a Zilog Z800 16 bit CPU upgrade board to replace the Z80 8 bit CPU but this option was never released. In 1987 H.I. Tech produced an enhanced CPU board, the XLR8er,[21] using the Hitachi HD64180 Z80-compatible processor. It runs with a 6.144 MHz clock rate and adds 256KB of memory that could be used as a RAM disk. When combined with the upper 64KB of Model 4 banked RAM, a ramdisk of up to 384KB can be mounted. Later, software was developed that can access XLR8er RAM as standard TRSDOS 32KB banks through the @BANK supervisor call. This makes the extra memory accessible to standard TRSDOS applications coded to use banked RAM. The Hitachi CPU also executes many Z80 instructions in fewer clock cycles than the Zilog chip; 8Mhz performance was claimed but in reality most software realize a performance improvement of only 25 to 30 percent.[note 3]

The Model 4 include a sound generator, a first for the TRS-80 line as the Model I/III require the user to output sounds to the cassette port, which is then connected to a stereo amplifier for sound output. However its sound capabilities are extremely limited, with just seven tones that can be produced, ranging from C to G♭ on the musical scale, and there is no E. Each tone had 32 different durations it can be sounded for. TRSDOS includes a filter program, CLICK/FLT, that beeps the speaker each time a key was pressed, to provide audible feedback for the typist. One Radio Shack spokesman described the Model 4's sound generator as "being intended for business alerts".

TRSDOS 6 provides utilities and filters for the Model 4's new hardware features.[note 4]

SPOOL, a print spooler that runs as a background task while other applications are in use, can use the extra 64KB RAM, base 64KB, or disk as its buffer.

MEMDISK, a RAM disk, can use either or both RAM banks, or the base 64KB. The extra 64K can hold TRSDOS, freeing all floppy drives for data diskettes. Installing the system on RAM disk also speeds the computer's operation greatly, as system overlays load from memory rather than from disk.[6] Space on a system Memdisk can be saved by omitting the file SYS0/SYS, which contains the portion of TRSDOS which is resident in RAM once the computer is booted. Users not using an Extended Command Interpreter may also omit SYS13/SYS, which is a dummy file if an ECI is not installed. A SYSRES command is also provided which specifies overlay modules resident in the main 64KB, permitting more free disk space on the system diskette in drive zero, which also enlarges the free space available on a system Memdisk.

The BACKUP utility is versatile, if sometimes confusing for beginners because of its many parameters. This is eased somewhat since its parameter structure is the same as that supported by the DIRectory command, which will show the files that would be selected by BACKUP. It is especially capable of efficiently backing up diskettes with single-drive Model 4s, prompting the user when to switch disks in the drive. A fast mirror-image copy function (entire tracks are read and written at a time) is supported when the source and destination diskettes share like formats. This occurs automatically if the disks are alike, but a backup-by-files operation can be forced using the file wildcard character (dollar sign). This has the advantage of defragmenting files that are scattered over the disk's surface(s) in multiple extents. BACKUP respects the TRSDOS Allocation Flag (A Flag), which specifies the starting cylinder to be used for file copying. This capability permits the user to advantageously place frequently-accessed files (especially the system /SYS files) close to the directory cylinder, thus speeding disk I/O and reducing physical wear and tear. (The A Flag can be displayed and changed via the MEMORY command).

One disadvantage to BACKUP is the disk space it occupies: two granules, or three kilobytes on a standard floppy disk. The built-in COPY library command resident in SYS6/SYS provides the most frequently used capabilities, and has the advantage of supporting TRSDOS's data redirection capabilities.

A FORMS filter makes tailored printouts possible for applications lacking capability for formatted printing. A keyboard filter, Keystroke Multiplier, lets the user define macro strings to the CLEAR and ALPHA keys. A simple (non-symbolic) machine language debugger resides in SYS5/SYS, and an extended command set for this resides in SYS9/SYS (these two files may be deleted for those users not needing the debugger). TRSDOS has an @DEBUG SVC available, which a programmer can insert or patch into a program to invoke the debugger under programmed control. Job Control Language serves as the equivalent to MS-DOS's batch processor. A JOBLOG facility records all TRSDOS commands issued. A capable terminal program, COMM/CMD, services the RS-232 serial port. TRSDOS 6, like previous versions, is supplied with a PATCH utility which allows non-programmers to install machine code program modifications.

The video display RAM and keyboard matrix are not memory-mapped as on the previous Models I and III. Rather, whenever a program called functions requiring video RAM to be modified, or the keyboard matrix to be read, TRSDOS executes code that switches out (made inaccessible) the uppermost three kilobytes from the Z80 address space (hexadecimal F400 - FFFF). This is replaced with the keyboard matrix from the address range F400 - F7FF, and the 2 KB static RAM of video memory from F800 - FFFF. The video display requires only 1920 bytes to render the 80×24 text screen; the remaining memory holds the typeahead buffer and the code that accessed it. While this frees 3 KB of Z80 address space for programs, it is marginally slower than writing directly into video RAM. This banked keyboard/video address space is compatible with the external 32 KB memory banks; it is not necessary to switch in Bank 0, the Z80's upper 32KB of base memory, to access the keyboard or video memory. However, some third-party memory expansion kits do not allow this.

The monitor is significantly dimmer in 80×24 mode than in 64×16, as fewer scan lines draw each character.[6] The video hardware supports characters in reverse video and Model I/III style block graphics. Due to the different screen resolution (640x240 pixels rather than 512x192 pixels), the block graphics characters are not of uniform height. The ten vertical pixels in each character cell are divided into three graphic blocks, the upper two of which were four pixels high, and the bottom graphics block takes the remaining two vertical pixels. These irregular graphic blocks make video games in Model 4 mode unattractive; game programmers prefer running in Model III video mode. A 40 character wide display mode is available using a double-width raster image. By manipulating the video hardware in machine code, the Model III video modes (64 and 32 character columns with 16 rows) are available as well. An alternate character set is available which included the entire Greek alphabet and special symbols. The TRSDOS 6 video driver (system device ✶DO) supports scroll protection up to seven video rows, and space compression codes. The user can change the system cursor character with the SYSTEM (BLINK) command, and user programs can change it dynamically. The system video driver provides a supervisor call, @VDCTL (video control), that permits direct manipulation of video RAM. The entire video screen, or any single row, can be copied into a buffer supplied by the caller, or the buffer can be transferred to video RAM. Though this bypasses the ✶DO display output device chain, the calling routine can fetch the SVC's address out of the SVC table and re-vector it to supply data to the display driver.

One peculiarity of the Model 4 video driver is the way ASCII carriage return characters (13 decimal or 0D hex) are handled. When this character code is sent to the ✶DO driver the system cursor is both returned to column zero and a linefeed is issued, which advances the cursor to the next line (causing the screen to scroll if the cursor was at the screen's bottom at row 23). This is contrary to other Z80 computers running the CP/M OS, and the industry-standard IBM PC DOS and MS-DOS OSes. These do not issue automatic linefeeds, which must be supplied by the software application separately. The result of this peculiarity of the Model 4 video driver is that files imported from these other OSes with single-spaced text will appear double-spaced on the Model 4 display. Many users processed text files like these with a utility that would strip out the redundant ASCII linefeed characters (10 decimal or 0A hex). In large text files this may save thousands of bytes, the reason LSI chose to handle carriage returns this way. This peculiarity also affects the way TRS-80 printers output single-spaced text imported from the other OSes. One convenience that Tandy/Radio Shack built into their own printers (DMP and DWP series) was a DIP switch to handle carriage returns either as standard TRS-80 CR+LF characters, or IBM standard carriage returns without the automatic linefeeds. Yet another effect is the display of text files with discrete CR+LF pairs received through the RS-232 serial port, usually from a remote host connected via a modem and a telephone line. Most Model 4 terminal programs have a provision for screening out redundant linefeeds before the text is sent to the ✶DO driver; for those that don't a simple filter program can be attached to the device chain to perform this task.

An optional expansion board permits the Model 4 to display 640×240 or 512×192 high-resolution monochrome graphics. The Radio Shack Model 4 hi-res graphics board sold for $249 and include a modified version of Microsoft BASIC (called BasicG) with commands for drawing basic geometric shapes and manipulating arrays in graphics RAM. The graphics screen memory is separate from the usual character screen, and the two can be displayed together or separately. Micro Labs of Richardson, Texas sold a popular compatible graphics board for $199. It comes with GBasic, its own BASIC with graphics commands. Whereas the Radio Shack board has 32KB of graphics RAM, the Micro Labs board has 20KB. Since only 19,200 bytes are required to render a 640 x 240 pixel screen, the additional RAM on the Radio Shack board is available for windowing the viewable screen around a larger virtual area. The graphics RAM is accessible through four Z80 I/O ports and is especially easy to access using the Z80 block I/O instructions for uses other than graphics. For example, the public domain utility Grafdisk adds the graphics memory to regular banked memory to create a larger TRSDOS ramdisk (96K on the Radio Shack board).

The keyboard adds CTRL, ⇪ Caps Lock, and three function keys to the Model III keyboard.[6] The system keyboard driver produces character codes for all the alphanumeric keys and function keys pressed in combination with the CLEAR and ⇧ Shift+CLEAR keys. Though the keyboard lacks a Tab ↹ key, tabbing is performed with CLEAR+RIGHT-ARROW. Pausing a display, such as with DIR or LIST, is done with ⇧ Shift+@. As with previous TRS-80s, there is no ESC key, but rather there is a dedicated BREAK key which performs the same function. Special punctuation symbols such as [ or { are entered with combinations of CLEAR or ⇧ Shift+CLEAR and ordinary punctuation keys such as ( or /. The video display can be dumped to the printer by pressing CTRL+:. CTRL+R repeats the last TRSDOS command. TRSDOS supports a typeahead feature with an 80 character key buffer. It uses the TRSDOS event tasker, so operations that disable Z80 interrupts (such as floppy disk access) can result in missed keystrokes.

Both Model 4 BASIC and Model III BASIC are provided. Model 4 BASIC is largely compatible with the older language. All CMD"A"-"Z" functions were removed; most, not all, are available with different syntax. New features support variable names longer than two characters, WHILE...WEND loop structures, program chaining,[6] and user defined functions (DEF FN). Some features from the Model III BASIC were dropped such as the ability to compress BASIC statements by omitting spaces (this feature, also found in Commodore BASIC, reduced the memory footprint of programs). It also lacks the commands for setting, resetting, and testing graphics blocks on the display.

For Model III BASIC programmers Model 4 BASIC has two disadvantages. First, variable names have to be separated from BASIC keywords with spaces, unlike Model III Disk BASIC which permits them to be run together; the Model 4 interpreter's variable names can be up to 40 characters in length, and the Model III interpreter's variable names have to be one or two characters (the interpreter accepts longer names but only the first two characters are significant for uniquely identifying the variable). The Model 4 version of BASIC therefore has to search for the delimiting space to find the end of the variable name. What this means for the programmer converting old Model III program into Model 4 BASIC is tedious editing, because Model III programmers, to save memory and speed execution, typically compact their code by using two-character variable names and eliminating spaces separating variables and keywords. The second disadvantage is that Model 4 BASIC lacks the command available in Model III BASIC for sorting arrays (CMD"O"). This is a problem for programmers maintaining code for business applications, and caused many BASIC coders to write a replacement capability or purchase third-party software. Another solution preferred by some is to continue running their programs in Model III mode under Model III BASIC, but activating the Model 4's faster speed, larger video display, and extra keys by manipulating its hardware with machine code. For those programming in languages never updated for the Model 4, this is their only option for accessing the new Model 4 features (Microsoft eventually updated its language products for the Model 4).

Tandy sold 71,000 Model 4 computers in 1984.[22] BYTE in October 1983 noted the lack of native software, but praised the Model 4's backwards compatibility and TRSDOS 6's new features. The magazine concluded that the Model 4 "provides a lot of flexible computing power ... Radio Shack has a guaranteed winner".[6] Creative Computing chose the Model 4 as the best desktop computer under $2000 for 1984, stating that the $1299 price for a system with two disk drives was "a real bargain".[23]

InfoWorld in 1983 predicted that the 4P would be a "smashing success" as a "substantial improvement" on the Model 4's video and keyboard. The magazine said that it was "truly a transportable computer" and approved of the "carefully thought-out mechanical design", not too large or small. Although criticizing the computer's lack of advanced documentation or double-sided drives, InfoWorld concluded that the 4P "is an outstanding product at an excellent price".[26]