I started my first code development on a Plus/4 in late 1986. After I saw a HomeLab 3 (made in Hungary, U880 - GDR made Z80 compatible proc, B/W, 16K), I started writing demos and other software for the Plus/4 machine I owned. It actually wasn't that strange to see demo groups sprout up for all kinds of machines, including the Plus/4. All over, there were groups and individuals, writing software while trying to keep the flame lit for each machine. In fact, I know people currently working in groups writing for the Plus/4 in Hungary, Germany, and as far away as Alaska.
Let's discuss the Text Editor (TED) IC and its environment. This DIL-48 IC was designed specifically for the 264 series of machines, which initially included the CV364 and the 264, evolving into the Plus/4, C16, and C116 machines. Unlike the CIA or ACIA or other machines, this IC isn't well suited to any other system.
The TED contains all functions done by several chips in former Commodore computers. The TED is a complete video-interface and composite video signal generator, sound generator, keyboard input latch, timer, clock generator, memory manager and DRAM refresher in a single IC. It can address the full memory map of the 264 series machines, and it generates the RAS', CAS', and MUX signals for the DRAM memory used in that series. For ROM, it generates the chip select (CS) lines, depending on the state of the internal registers. So, in addition to all the above duties, the TED IC is a simplistic MMU as well.
Of course, there are differences. TED does not contain sprite support.
To offset this omission, the TED chip can select 8 intensities for each of the 16 supported colors, giving 121 colors (the 8 shades of black are all black). Other features include a hardware cursor, hardware text blinking, and hardware inverse character support. Character sets, screen and color memory, and graphics bitplanes can be addressed directly, without additional logic as found on the C64. In fact, even RAM/ROM selection requires change of a single bit.
Character modes need $800 bytes of RAM for screen and color memory. The first $400 bytes act as color memory (the memory permanently located at $d800 on the C64), with the lower 4 bits containing color codes, exactly as found on the 64. Bits 4-6 denote the intensity level of the color, while the high bit select flashing/no-flashing attributes. The other $400 bytes contain the screen codes for the displayed characters. If hardware character inversion is selected, the lower 7 bits hold the screen code and the high bit selects inversion for the character. If character inversion is not selected, all 8 bits denote the screen code. Extended Color Mode (ECM) and Multi Color Mode (MCM) modes work exactly as described on the 64. While these two modes are in effect, inversion and blinking are disabled.
Things get a bit more complex in graphics mode (pun unintentional). In graphics mode, the bitplane occupies $2000 bytes and is handled just like a VIC-II biplane. The colors are handled differently. $800 bytes are needed for color memory, which is laid out in $400 bytes of intensity memory and $400 bytes of color memory. An "off" bit in the bitplane uses the lowest nybble of the appropriate color memory location as the color and retrieves the intensity from bits 4-6 of the appropriate intensity memory location. For an "on" bit, the color is taken from the high nybble of the appropriate color memory location, while the intensity is taken from bits 0-2 of the intensity memory location. Bits 3 and 7 in intensity memory are unused.
In multicolor mode, differences abound. The 64's VIC-II enabled one to utilize 3 different colors in each 8x8 cell and a single background. The TED simply cannot accomplish this due to the lack of adequate color memory. So, TED allows only 2 varying colors per 8x8 cell. Those colors are chosen from the palette of 121. The remaining 2 colors are chosen for the entire screen, again from the 121 color palette. The mapping is as follows:
00 background color 01 same as "off" color in hires mode 10 same as "on" color in hires mode 11 another "background" color
The TED IC is able to generate both PAL and NTSC compatible signals from a single IC. Only the crystal need be changed to go from one standard to the other. In PAL mode, there are 312 lines shown, while NTSC only has 262 lines of display. The line synchronization is the same in either PAL or NTSC mode. It's always 57 clock cycles per rasterline. The TED divides the supplied crystal frequency by 20 for PAL display and by 16 for NTSC.
For the serious video programmer, raster interrupts are implemented as on the VIC-II. However, the 0 line of the register corresponds to the first line of the character screen area, not the top of the border. In addition, the current raster line can be read from TED registers. You can modify the counter as well. Doing so will most likely affect the screen display. As a bonus, the horizontal location of the raster can be read and modified in the same way. Unfortunately, these registers provide the basis for most effects, as the TED can't handle sprites.
To play samples, the TED can switch the sound generators to constant level outputs. D/A is then done by changing the volume register setting. Each generator can generate frequencies from 100Hz to 23kHz.
Since it already does almost everything else, it's not unusual to notice that the TED handles the keyboard matrix. A simple 8-bit input latch handles keyboard interfacing.
As noted above, a single bit in the register space will page ROM or RAM into the upper 32kB of the address map. Since the TED knows what is paged in at all times, it knows what to output to access the memory locations in this area.
By the way, catch FLI ED. V1.0. Its info file may contain some more about TED's screen-handling. It may be retrieved as ftp://ftp.funet.fi/pub/cbm/plus4/tlc/cns.lzh
Register Description -------- ----------- $ff00- $ff01: Counter #01. It always starts to decrement from the last written value into it. $ff02- $ff03: Counter #02. It runs freely from $ffff. $ff04- $ff05: Counter #03. Same as above. $ff06 : Mostly the same as VIC's $d011. Bit 0,1,2 : Vertical smooth-scrolling Bit 3 : 24/25 rows screen Bit 4 : Blank screen Bit 5 : Bitplane mode Bit 6 : Enhanced color mode Bit 7 : TED's internal test, it should be 0. $ff07 : Most similar VIC-reg is $d016. Bit 0,1,2 : Horizontal smooth-scrolling Bit 3 : 40/38 columns screen Bit 4 : Multicolor mode Bit 5 : TED stop. If set, the TED stops it's counters and screen-generating, only single clock and refresh cycles remain. Bit 6 : PAL/NTSC. 0:PAL, 1:NTSC Bit 7 : Disable reverse mode. If 0, we got 128 characters and higmost bit tells if the character should appear in inverse. If set, no inverse mode but 256 characters. $ff08 : Keyboard input latch. Giving a strobe - writing to the register, the latch stores the values of the input-lines. Then, we can read them from this register. $ff09 : Interrupt request register. When a counter sends want to send an IRQ, it's bit will appear as a 0; then, if the IRQ was caused then highmost bit is set. Bit 0 : Unused Bit 1 : Raster-counter Bit 2 : Lightpen. Not implemented. Bit 3 : Counter #1 Bit 4 : Counter #2 Bit 5 : Unused Bit 6 : Counter #3 Bit 7 : Interrupt occured. This bit is set when an IRQ was enabled and therefore, the IRQ was sent to the processor. Physically, this is the negated level of the TED's IRQ output. The IRQ should be deleted with writing the register-value back after accepting an interrupt. $ff0a : Interrupt mask register. These bits could be used to disable and enable interrupt-sources. When a place is set to 1, that will be able to cause an interrupt to the processor. If not, the sign of the interrupt request will only be appear in the above register. Bit 0 : 9th bit of $ff0b (see there) Bit 1 : Raster-counter Bit 2 : Lightpen. Not implemented. Bit 3 : Counter #1 Bit 4 : Counter #2 Bit 5 : Unused Bit 6 : Counter #3 Bit 7 : Unused $ff0b : Raster interrupt register. Same as $d012 when writing; it stores the position of occuring raster interrupt. Higmost bit is in $ff0a's 0. bit. $ff0c,$ff0d : Hardware-cursor position (10 bits). Lower bits: $ff0d, higher 2 bits in $ff0c's 0. and 1. places. Beyond 1000 the cursor is not seeable. $ff0e : This reg is the first sound-source's frq-value's lowmost 8 bit. More 2 bits are in $ff10's 0. and 1. places. $ff0f : 2nd. source, lowmost 8 bits. More 2 bits in $ff12, 0. and 1. places. The soundregister-value can be calculated as reg=1024-(111860.781/frq[Hz]) (NTSC) reg=1024-(111840.45 /frq[Hz]) (PAL) $ff10 : 1st. sound-source, higmost 2 bits. 2-7 bits are unused. $ff11 : Sound control register. Bit 0-3 : Volume. Maximum value is 8. Bit 4 : Sound #1 on/off. Bit 5 : Sound #2 squarewave on/off. Bit 6 : Sound #2 noise on/off. If You set both, the square will sound. Bit 7 : D/A mode. See above for more. $ff12 : Bit 0,1 : 2nd sound-source, highmost bits. Bit 2 : Character generator in ROM or RAM. When set, TED will enable ROM when trying to get data from the charactergenerator to build screen. Else, it will give out control-signals to the DRAM's. Bit 3,4,5 : These bits tell, where to find bitplane in the memory when using bitplane-mode. TED assumes them as A15,A14 and A13 bits. So, the bitplanes can be switched as 8K pages, anywhere in the 64K. Bit 6-7 : Unused. $ff13 Bit 0 : A sign to having control about memory paging. This bit always sets to 1 when ROM is active over $8000. Else, it will be 0. READ ONLY. Bit 1 : Force single clock mode. Then, TED will disable to generate twiee clock. Bit 2-7 : Charactergenerator. Bit 7 corresponds to A15, 6 to A14 and so on. This value shows and sets the start of the charactergenerator. It can be paged as $400 bytes. Use with addition of $ff12-2.bit. $ff14 Bit 0-2 : Unused Bit 3-7 : Start of the video-ram. Bit 7 also corresponds to the A15 line as above. So, video-ram is mappable as $800 bytes - 2K. The above $ff12-2.bit doesn't affect this, but the actual RAM/ROM mapping (see at $ff3e/$ff3f and $ff13/0) does. $ff15 : Background. Lower bits contain color-code, higher 3 luminance and higmost is ignored. $ff16 : Color-reg 1 $ff17 : Color-reg 2 $ff18 : Color reg 3. This and the above are used in ECM and MCM modes. $ff19 : Border. All color registers use codes as described in $ff15. $ff1a : Bit 0-1 : Higmost bits of the next $ff1b Bit 2-7 : Unused $ff1b : Actual character-position. Higmost bits in the above register. TED counts the characters that it had fetched and put out to the screen. The number is increasing by 40 after every characterline (8 rasterline). $ff1c : Bit 0 : Higmost bit of $ff1d Bit 1-7 : Unused $ff1d : Actual position of vertical scanning. Higmost bit is in $ff1c. Read/Writeable! $ff1e : Actual position of horizontal scanning. R/W!. Lowmost bit is unused. It contains the TED's internal counter's highmost 8 bits. So, it increases 4 with every character. When writing, it seems to put the value to a functionally different register (writing back a reading value in right time affects the screen). $ff1f : Bit 0,1,2 : Actual vertical scanning-line in a character-row. R/W!. Bit 3-6 : Flashing counter. It's value increases with every frame, and TED fits it's flashing feature to this register's reaching to 15. Bit 7 : Unused $ff3e : Switching to ROM. A writing statement to this address will cause to turn on the ROM between $8000-$ffff. It's an other matter, which one; this time, only sure thing that it'll give CS signals instead of RAS', CAS' and MUX. See $ff13/0 and $ff14 $ff3f : Switching to RAM. The opposite of the above.