Taking the Wraps off the 34020

A trio of chips that puts workstation power on a small board

Source: BYTE Magazine, September 1988 (link), Page 257+
Authors: Ron Peterson, Carrell R. Killebrew Jr., Tom Albers, and Karl Guttag

Ed. More editing needs to be done due to the seeming original editing mistakes that were caused / hidden by placing the text in flowing columns. Also, having columns of text above, to the side, or below illustrations just makes it harder to keep continuity...

The new TMS34020 32-bit graphics microprocessor (the '20) is well suited to workstations and personal computers requiring highly interactive user interfaces. The '20 is also suited to laser printers, which are becoming more performance hungry, with requirements for on-the-fly font compilation, and the increased complexity of page-description languages, which are constantly demanding more performance.

Data and image compression such as facsimile and CD-ROM are other areas where the '20 will find ready applications, since fast bit addressable processors provide inherent advantages over general-purpose processors for performing the Huffman-type encoding and decoding necessary for CCITT Groups 3 and 4 fax standards. In fact, for TMS340 Graphics System Processors (GSPs), fax-type compression and decompression is just another program (i.e., no additional hardware is required).

A Chip off the Old Block

The '20 is the newest member of Texas Instruments' (TI) TMS340 GSP family. Depending on the instruction mix, it's between 6 and 50 times faster in key graphics operations than its predecessor, the TMS34010 (the '10). The '20 runs at 10 million instructions per second (MIPS) when executing from its 512-byte instruction cache. It's designed to connect directly to a second '20 as well as to the 40-rnillion-floating-point -operation-per second (MFLOPS) TMS34082 graphics floating-point coprocessor (the '82 FPU). The '20 has instructions that can perform pixel- or bit-aligned block transfers at 142 megabits per second, and when using the TMS44C251 1-megabit video RAM (the 44C251 VRAM), the '20 can execute fills at up to 1.136 gigabits per second.

Figure 1: The TMS34020 block diagram

Note the coprocessor interface and the built-in host attachment capabilities.

The '20 has all the architectural features that define TI's TMS340 GSP family (see figure 1). These features include a host-bus interface, a local-bus interface with semiautonomous memory controller, a display-control interface and registers, instruction cache, 31 32-bit registers, single-cycle general-purpose instructions, and multiple-cycle graphics instructions. (For details on the '10, see "The TMS34010 Graphics System Processor" in the December 1986 BYTE.)

The '82 FPU is based on the same design used by many bit-slice graphics workstation engines. It directly connects to the '20's coprocessor interface and has external expansion for both microcode and data.

The '20 has also been designed to take full advantage of the features of the 44C251 1-megabit VRAM. These improvements allow faster fills, masked operations, and text without resorting to the expense of the wide data buses used in many workstations.

Theory of Evolution

The concept of a graphics engine designed around a fully programmable microprocessor is what made the '10 different from other graphics chips and has led to the TMS340 family's success. The device differs from a normal microprocessor in the special hardware and microcode included to support bitmapped graphics operations. It also integrates graphics system-control features for the CRT, dynamic RAM (DRAM), and VRAM onto one device. The underlying goal in this design was to let the flexibility of bit-mapped graphics be matched by the flexibility of a graphics processor.

The definition of the '20 began during final development of the '10. Features that were too expensive or that would cause major schedule delays on the '10 were slated for implementation on the '20. The first issue was to obtain an overall speed improvement over the '10. This began with the obvious expansion of the '10's 32-bit internal and 16-bit external busing to a full32 bits for both on the '20.

With the addition of 32-bit pixel and 32-bit external bus support, several graphics features have been enhanced on the '20. Pixel-formatted registers are expanded from 16 to 32 bits in significance; these include the color-expand registers and the plane-masking register. Other enhancements include improving the processor cycle time and utilizing the fast page mode of DRAM. Also, the instruction cache was doubled to 512 bytes on the '20 to reduce cache misses and let larger algorithms fit into the cache.

New capabilities were also added. Three-operand block transfers were needed by both laser-printer manufacturers dealing with large textured objects and system developers building extensive window environments. Many system designers using the '10 had requested better support for direct host access to the TMS340's memory, and some of the more sophisticated applications required the graphics processor to do operations in the host 's memory space.

As a result, the host interface was totally revamped for the '20. Better CRT timing control was added to the display controller to enable broadcast-quality RS170 National Television System Committee (NTSC) timing. New XY addressing support and improved VRAM control were also added to pack together frame-buffer scan lines for better memory utilization. The '20's VRAM support takes advantage of the 1-megabit VRAM's split shift register as well as the older (and less efficient) 256K-bit VRAM's midline reloading.

The '20 has enhanced the XY addressing support of the '10 to include direct support for pitches other than binary for the XY address space. (Pitch is the distance between pixels on corresponding lines.) Both pitches that are the sum of two powers of 2 (A + B where both A and B are powers of 2) and completely arbitrary pitches are supported. This makes the coding of output routines into linear memory and instruction support for packed screen organizations much easier. In addition, both X and Y values on the '20 are treated as signed 16-bit numbers.

The definition of the '20 has gone far beyond its own chip boundaries to include the development of an extremely fast graphics floating-point coprocessor, the '82. This processor is 30 to 100 times faster than the typical microcomputer's FPU.

The Class of '82  [Text Box]

The 34082 is a high-performance graphics floating-point coprocessor developed exclusively for the 34020. Integrating a rnicrosequencer, register file, floating-point ALU, and multiplier onto one chip (see figure A), and supporting high-level graphics instructions, the '82 can execute floating point operations at up to 40 MFLOPS.

Figure A: TMS34082 floating-point coprocessor block diagram

Notice the 22 64-bit registers internal to the chip.

The register file on the '82 consists of 22 64-bit registers. The file has 10 A-file registers and 10 B-file registers for data storage and two C-file registers for feedback. This large register file lets complex algorithms store intermediate results internal to the device, thereby reducing the memory cycles necessary to reload these operands for future calculations. The device also incorporates a local bus for storing up to 64K 32-bit words for data and microcode expansion. This expansion memory lets you develop your own functions or store data local to the '82.

Using the '20's coprocessor interface, the '82 directly interfaces to the '20 with no "glue" logic, letting the '82 be either initially designed into the system or socketed for later addition. This interface allows connecting up to four '82s to the '20. The '20 provides a general coprocessor interface to let the external implementation of advanced functions improve performance. The '82 is one example of a coprocessor taking advantage of this interface for performance increases. The interface consists of five basic cycles for data communication and control:

  1. Move data from '20 register to coprocessor.
  2. Move data from coprocessor to '20 register.
  3. Move data from memory to coprocessor (indirect '20 register move).
  4. Move data from coprocessor to memory (indirect '20 register move).
  5. Execute coprocessor command.

Not only does it support fully compatible IEEE Standard 754-1985 additions, multiplications, divisions, square roots, and comparisons, the '82 has instructions for complex graphics math.
Table A gives a complete list of instructions. And the expansion capability of the '82 allows for the creation of an indefinite number of user-defined functions.

The '20 project team also worked closely with TI's memory-design engineers to allow the '20 to distribute some processing within the new 1-megabit VRAM, the 44C251. (See the text box "Making Videos" on page 272.) Coordination between the development programs and the resulting cooperation between these three chips enables simpler system design and higher performance.

Making Videos [Text Box]

The video RAM (VRAM) is a dynamic RAM (DRAM) modified to let it transfer a large number of bits from the memory array to a separate internal serial-shift register. Prior to VRAMs, the majority of the available bandwidth in conventional frame buffers (designed around DRAMs) was used for display refreshing. This left virtually no time for a processor to update the display.

After making the transfer, the contents of the shift register are independently shifted out to the display. As a result, the random port of the VRAM allows unimpeded access to the graphics processor. With the increasing resolutions of graphics displays and denser memory devices, the VRAM is becoming indispensable to frame-buffer design.

The original 64K-bit VRAM was organized as a 64K-by-1-bit device and had a single 256-bit shift register with four tap points (the starting column position for the shift register). Memory array-to-shift-register transfer cycles could be made only during horizontal retrace when the serial clock to the shift registers was stopped.

The next-generation VRAM was a 256K-bit device that combined the functionality of four 64K-bit VRAMs on a single chip. The device was organized into a 64K-by-4-bit device and contained four 256-bit "serial-shift" registers. A static RAM and counter emulated the shift-register function of its predecessor, allowing the shift function to start from any tap point.

Transfers in the middle of a line were enabled by changing the timing of the shift-register-to-memory transfer. This "midline reload" allowed designs to use fewer VRAMs for certain frame-buffer sizes, but the timing was so critical that few systems could take advantage of it. Furthermore, various manufacturers experimented with features including fast page mode, Boolean functions, and alternate shift-register timing modes.

The latest VRAM generation is a 1-megabit device that has several architectural enhancements to improve performance. The organization is 256K-by-4-bit with four 512-bit shift registers. Some manufacturers plan an alternative 128K-by-8-bit VRAM. It's important to note that when graphics architectures use data buses wider than 128 bits, some memory devices will be only partially used, because they need extra devices just to reach the required data-bus width due to the "deeper" organization of the by-4 memories. To compensate for the increase in the depth of the memory, the 1-megabit VRAM includes features to improve its performance without requiring wider buses.

Most 1-megabit VRAMs support the block-write mode (BWM), which gives a performance improvement of four times or more on key graphics operations, such as fills and color-text generation. Fast page-mode accesses (common on 1-megabit DRAM) further improve memory bandwidth. Some 1-megabit VRAMs will support "persistent write per bit, "which lets color masking be "locked in" on the VRAMs, reducing read-modify-write (RMW) cycles. The 1-megabit VRAM will also support "split-shift-register transfers," simplifying the timing and control circuitry necessary for real-time reloading of the serial-shift register. The split shift registers provide memory savings in non-binary-powered display resolutions.

BWM is the most significant new architectural feature on the 1-megabit VRAM. On the 256K-by-4-bit VRAM, an internal 4-bit color register is loaded from the 4-bit-wide data bus during a special write cycle. At 4 bits per pixel, this latch contains the color value to be written; at 8 bits per pixel, two VRAMs are concatenated to form an 8-bit register.

When the VRAM's control signals select BWM, the two least-significant bits of the address are ignored, and the four "data pins" control the writing of the contents of the color register into any or all of four consecutive locations (a "block") in memory. A "1" on a data line initiates the writing of the color register value, while a "0" prevents that writing to the corresponding position in the block. With four data lines controlling the four 4-bit memory cells , the net effect is an effective increase of four times in write bandwidth. The ability to support individual pixel control in BWM allows this feature to be used for rendering color text and area filling.

A Marriage of Convenience

Since the '20 and the 44C251 1-megabit VRAM definitions were coordinated during development, the '20 supports performance and system features added to the 44C251. The designers of the '20 had the advantage of knowing and influencing the definition of the VRAM, so the two architectures fit together.

The importance of that joint definition can be seen most clearly with the blockwrite mode (BWM). The 44C251 has four banks of 64K -by-4-bit memory, for a total of 16 bits wide internally (see figure 2a).

Figure 2a: TMS44C251 VRAM internals

Notice the four banks of64K-by-4-bit memory,for a total internal width of 16 bits.

In normal mode, the two least significant bits of the address select one of four banks-only one can be selected at a time-and the four data lines provide the data. In BWM, the four data lines used enable writing to any and all combinations of the four banks within each 44C251, and the special color-latch data is multiplexed to provide the data when it's written. The '20 controls the 44C251 to expand the 32-bit data path to 128 bits wide inside the VRAM (see figure 2b).

Figure 2b: 44C251 and its connection to the '20

Notice how the '20 expands the 32-bit data path to 128 bits wide inside the VRAM.

The BWM function is modeled after the TMS340 family's "binary to color expand" function. Color expansion works by turning data into control signals that select whether or not to write a color. Since each data line controls a multiple-bit color value, we use the term "color expansion."

Both the ' 20 and the '10 have flexible color expanders that turn 1-bit-per-pixel shape information into 1, 2, 4, 8, or 16 bits per pixel. The '20 extends this function to 32 bits per pixel. This function, widely used in bit-mapped color graphics, would be very tedious without hardware help

Although the BWM has the potential to give an additional performance increase of four times or more for fills and color text , it requires special addressing and control- and data-mapping hardware built into the '20. The mapping hardware makes writing pixels into the memory consistent with how they would be written without BWM and can be thought of as the "key" that unlocks the block-write feature.

When using the BWM, you can think of each 44C251 VRAM as having four 1- to 4-bit fixed color expanders in it; in other words, each memory can expand 4 bits of incoming data or control information into 16 bits. By combining two VRAMs (to get an 8-bit-wide color latch and expander), you can get four 1- to 8-bit expanders.

By using the '20's color-expander hardware plus additional on-chip hardware to generate the control signals necessary to interface to the VRAMs, the '20's 32-bit data bus is effectively expanded to 128 bits during BWM cycles. The '20 also supports the color-expand functions for complex graphics math. Table A gives a complete list of instructions. And the expansion capability of the '82 allows for the creation of an indefinite number of user-defined functions. function for memories without BWM, which still gives an improvement that is roughly 10 times that of microprocessors without color-expand hardware.

Table A: Capabilities of the 34082 FPU instruction set

Two special instructions were added to the '20 to take advantage of the BWM. VFILL is used for fast solid filling. The VBLT instruction reads binary shape information and controls the VRAM for text and pattern color expansion.

Another area of improvement is in the plane-masking function. The 44C251's "persistent write per bit" speeds up this function. The '20 doesn't have to do read-modify -write (RMW) cycles to perform the masking operation.

By combining the wider data bus, fast page mode, faster processor cycle , and BWM support on the '20, you get a performance advantage of up to 50 times over the ' 10 when writing color text. Remarkably, the '10's internal color expand hardware already gave it a significant advantage over general-purpose processors at this function.

The 44C251's split shift register greatly simplifies the timing requirements for loading the shift register while it's still allowed to shift. The special control and timing are directly supported by the '20, removing the time-critical real-time reload headache from the system designer.

Strike Up the Band

The ' 20 significantly increases the system-bus bandwidth of the '10 due to improvements in three basic areas: a wider data bus (32 bits on the '20 versus 16 bits on the '10), exploiting enhanced page mode accesses on DRAMs and VRAMs, and a faster clock speed or state rate. The wider data-bus and page-mode accesses let the '20 fetch more instructions in less time on a cache miss. And, at a clock speed of 40 MHz, the '20 typically fetches 16 instructions in 900 nanoseconds, while a 50-MHz '10 fetches only3. Together, these improvements provide performance increases of three to seven times on the '20 when executing programs written for the '10.

The largest performance increases due to local-bus bandwidth improvements, however, come in the multiple-cycle data-manipulation instructions. The '20's FILLs, PIXBLTs, and BLOCK MOVEs have improved the most. For example, a fill operation on the '20 paints the screen in one-seventh the time required by the '10; you can get even more dramatic performance increases, up to 50 times that of the '10 , if you use the block-write capabilities of the 44C251 VRAM. For large data transfers, the '20 incorporates an eight-word (32 bits per word) first-in/first-out that scoops up the source data, modifies it, and returns it to the destination within local memory.

Another unique feature of the '20 is the four CAS- outputs (see figure 1) that support byte-write operations, thus avoiding RMW cycles. Many graphics and string operations require intensive byte manipulation provided by the '20's CAS- strobes. Furthermore, these strobes let the '20's line-drawing engine splatter the display with 5 million pixels in a single second (RMW cycles would reduce performance to less than 3 million pixels per second).

In addition, the '20 dynamically sizes to 16- and 32-bit-wide memories via the SIZE16- input; simultaneously, the '20 can dynamically change its column and address multiplexing to support memories of different array sizes in the same system without additional hardware.

Hosting the Affair

The '20 operates as either a host processor or a graphics processor attached to a host bus. The host interface on the '20 is completely transparent to the host system. When the '20's memory is mapped directly into the host system's memory map, the '20 functions as a DRAM controller.

On a host access to the '20's local memory, the host's address is latched into the '20. The address is then reformatted and presented on the LAD and RCA buses to the '20's local memory (see figure 1). Data is routed via external data buffers that are directly controlled by the '20.

The 38-pin interface supports burst rates of up to 20 megabytes per second in either block-transfer or random-access mode. The '20 provides option bits in the internal host control register to let the interface pipelining be configured for block (read or write) or RMW operations for maximum performance.

The '20 supports multiprocessing environments and can access host memory directly to improve system performance when accessing host resources. Built-in bus-fault and retry mechanisms handle address violations and bus-contention conditions. The '20 supports data: operations in big endian (data indexed from the most-significant data bit) and little endian (data indexed from the least-significant data bit) modes, thus removing swizzling (data remapping) operations.

Hold That Line  

The '20's Hold/Holda protocol not only lets another processor suspend the master GSP from operating on its local bus, but also lets other GSPs be directly wired onto the same local bus . Three signal lines-grant in (GI-), low-priority request (RL-), and high-priority request (RH-)-provide the handshake mechanism on this interface (see figure 1). These three lines are necessary to handle special problems associated with both graphics systems and DRAM- and VRAM-based systems.

In a typical Hold application, the '20 must reassert bus control if a shift-register-transfer (display-refresh) or memory refresh cycle is pending. Internal counters schedule latency of these events to provide maximum performance. Internal refresh queuing of up to 12 DRAM refresh cycles limits bus re-arbitration.

Two or more '20s can be directly wired together to increase system performance. Multiple GSPs synchronized to the same local clock can efficiently handshake over the three-wire interface, allowing control of the bus to be passed between processors on every local memory cycle. With the increased size of the instruction cache and the improved efficiency of instruction acquisition, a significant performance increase can be obtained by using this interface.

Learning the Rules

The '20 has built on the '10's full suite of general-purpose instructions and combined them with a rich set of application specific graphics instructions (see table 1). Note that the processors have a sufficient set of the necessary assembly language instructions to support a robust C language with direct field support.

Table 1: New 34020 instructions added to current set for TMS340 GSP family

Both the '10 and the '20 have direct support for operations on fields in memory. Since they are bit-addressable processors, fields in memory can start on any bit boundary; the instruction set further supports varying field widths (from 1 to 32 bits). This can be especially important in manipulating C "fields" and in the memory storage required for packed arrays in C. The TMS340 family's hardware support for fields augments these operations. (Pixels are a special case of regularly sized, packed fields in frame-buffer memory.)

Two-operand pixel-block-transfer instructions (PIXBLTs) perform 16 binary and 6 arithmetic raster operations on a pair of two-dimensional pixel arrays. The two-operand PIXBLTs are useful for manipulating and moving bounded objects and operating with solid color.

You can simulate more elaborate operations using multiple PIXBLTs, arithmetic raster operations, and the 10's binary-expand operations. Using the binary expand, fonts stored as bit patterns form color text (with transparency), and two-color patterned operations can be performed on the bit map. The flexibility of the PIXBLT instructions lets you build operations such as the 8514/A's compare operations, as well as foreground and background raster operations. In addition, you can build support for non-rectangular objects on top of two operand PIXBLTs.

The ' 20 extends the '10's PIXBLT instructions by adding a third operand, a binary-mask operand. While the '10 supplies hardware support for rectangular window regions and pixel blocks, many applications have begun to require control and rendering of non-rectangular window regions. The third mask array supports this feature on the '20. The mask is used to describe the window region on a pixel-by-pixel basis. This provides complete flexibility in describing a region: It can be filled, concave, convex, outlined, have holes, and so on.

The three-operand PIXBLT employs the binary-mask array to act on the first operand, the source-pixel array. This mask array selects individual pixels in the source array to be combined with the destination array (an independent operand). For text, the source array can be an arbitrary pattern (dithered or halftoned) describing the "color" or "shading" of the text (see figure 3). The mask array contains the description of the shape of the character, or glyph. The destination array is the displayable "canvas" onto which the character is painted.

Figure 3: The three-operand PIXBLT

In this way, the bits having a value of 1 in the mask select pixels (1, 2, 4, 8, 16, or 32 bits deep) in the source-array pattern to transfer to the destination location. There is the additional option of performing one of 22 raster operations on the selected source or destination pixels, as well as several transparency and plane-masking options.

See- Through Pixels

Raster operations on the TMS340 family include all the Boolean combinations of 2 bits and their complements, plus an arithmetic set. This latter set supports addition and subtraction of pixels as well as a unique pair of comparison operations. The first computes the maximum of the source and destination pixels. The second computes the minimum pixel value. These operations are useful for layering "planes" of data via the intelligent ordering of pixel indexes or colors. They have been used to "drag" sprite-like objects around the screen without corrupting background or foreground images. Pixels are combined through binary or arithmetic raster operations to form a resultant pixel.

Raster operations for the '20 contain several new transparency modes. Transparency is the notion that there are cases in which it is useful to inhibit the writing of new pixel data to the destination based on information within the actual operands of the raster operation. Thus, transparency circumvents the normal determination of what value is to be written to the destination pixel.

The four '20 transparency modes are transparency inhibited, transparency on resultant pixel = 0, transparency on source pixel = 0, and transparency on destination = COLORO . The first two are '1 0-compatible modes.

When transparency is inhibited, raster operations proceed as normal. For transparency on the resultant pixel equal to 0, the '20 checks the pixel value that results from the raster combination for zero-valued pixels. The zero value is then replaced with the original destination-pixel value so that the destination-pixel location is not changed.

For transparency on the source pixel equal to 0, the source-pixel value is checked before the raster operation for zero-valued pixels. Each zero value is identified so that the destination pixel retains its original value. This is useful for maintaining transparent pixels in the source-pixel array while using other '20 raster operations.

For transparency on the destination equal to COLORO, the destination-pixel value is checked before the raster operation for pixels that equal the "background" color (stored in the 32-bit COLORO register). Each background value pixel in the destination is not modified by the raster operation. This is useful for maintaining background pixels in the destination bit map, regardless of the value of the source pixel and which of the 22 raster operations are enabled.

Working on the Line

Several design areas on the '10 in the area of line support were targeted for enhancement on the '20. Among these were fast line draw, line initialization, and support for patterned lines and arrays.

To enhance line-draw performance, the '20 has added a fast line-draw instruction to the '10's Bresenham-algorithm line draw (see "Better Bit-Mapped Lines" in the March BYTE). This FLINE instruction performs the same arbitrary angle algorithm found on the '10, but it assumes pre-clipped endpoints as input. This allows the CPU to bypass the window-checking microcode and execute the heart of the line-draw algorithm at memory-bandwidth speeds (raster operations, plane masking, and transparency are all active).

To reduce overhead during line initialization, the LINIT instruction performs the necessary setup for the LINE instruction. LINIT speeds up and simplifies the operation overhead associated with line initialization. More important, LINIT allows larger algorithms utilizing the LINE instruction to fit into cache, thereby increasing drawing performance.

For enhanced patterned-drawing support, the '20 adds a horizontal-fill instruction, HFILL, which supports patterns described in a 32-bit internal register. The repeating pattern is taken from the register and rotated as pixels are drawn if the register contains a nonzero pattern. The PATTRN register contains the current drawing position in bit 0 at the end of the instruction.

But Can I Use It?

The '20 offers a full line of development tool support built on the '10 support environment. A full Kernighan and Ritchie optimizing C compiler utilizing many of the extended features of the TMS340 architecture is available. Supporting the C language environment is a full set of assembly language tools for building relocatable, ROMable, or all-RAM executable files. A TMS340 family common object-file-format (COFF) loader, C 110 package, and high -level-language debug support are planned for the '201

The '20 is object code-compatible with the '10. This means that math libraries, graphics libraries , graphics interfaces, and graphics-standards software are already available for the '20. These will be enhanced over time to use the extensions of the '20.

A Family Reunion

With the entrance of the '20 and '82, the TMS340 architecture addresses a very wide range of graphics applications. The '10's smaller package size and high level of system integration is ideally suited to high-volume cost-sensitive applications. The more powerful '20 provides an upwardly compatible migration path for '10 designs and meets the needs of higher performance systems.

By adding a TMS34082 Graphics Floating-Point Unit and TMS44C251 VRAM to a 34020-based system, a small board can have all the graphics and math capabilities of a high-end workstation.

Ron Peterson, Carrell R. Killebrew Jr., Tom Albers, and Karl Guttag are all members of Texas Instruments' design team for the 34020. They can be reached on BIX as "editors."

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