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COP420 4-Bit Processor

Contributed by Colin Appleby
Sunday, 31 August 2008

COP420 4-Bit Processor inside NewBrain Computer

TJ’s Workshop – COP Tips

Here are some tips for using the second processor inside the NewBrain Computer.

In addition to the Z80A which is its main processor, the NewBrain contains a National Semiconductor COP420 4-bit processor which is used as an intelligent I/O controller and manages the keyboard, line display, and tape I/O.

Communication between the two processors is handled via two page zero locations; COPCTL and COPST.  The COPCTL is a command byte which is written to the COP at interrupt time, and COPST is a series of flags which tell the Z80 what the COP has done, or is doing. There is also a third location, COPBUFF, which is used as a data buffer either to or from the COP. The COPCTL, COPST, and COPBUFF (the names are those used by the technical manual) are at 59, 60 and 61 respectively.

Unless tape I/O is going on, COPBUFF will normally contain the keyboard matrix value of the last key pressed. This value will remain until another key is pressed (it won’t change when you release the key).

The technical manual mentions the function of several of the commands, but is inexplicably coy about telling you what their values are … a deficiency which a little trial and error soon remedies.

It appears that only the top four bits of COPCTL are read by the COP, and those values for which I have been able to discover the function are as follows:

D0 - Do nothing (the default value, and the only one given in the technical manual).

80 - Perform tape I/O (I am uncertain of the details of the operation).

F0 - Rescan keyboard; this will rewrite COPBUFF with the value of any key currently pressed (it will not clear it if no key is being pressed).

A0 – Update line display from buffer area. The line display buffer runs from 62 - 77 inclusive, and is in reverse order (62 is the rightmost character).

So what use is all this? Well the rescan keyboard’ command allows us to tell when a key is being pressed, and also when it is released, which is otherwise impossible on the Newbrain. The following Basic subroutine will return the matrix value of any kept currently being held down (except STOP, REPEAT, the STOP can be detected by other means, of course). If more than one key is held down at once, only one will register, exactly which depends on the key.

The value is six bits wide, the top two bits give the status of SHIFT, CONTROL and GRAPHIC; SHIFT sets bit six, CONTROL bit seven, and GRAPHIC both bits, when pressed. The value returned from each key on the keyboard follows:

1000 a = PEEK(61): IF a = 0 RETURN

1010 POKE 61,0: POKE 59,240: RETURN

(This returns the key in a. It clears COPBUFF and rescans the keyboard if a key is being pressed).

Key      Value      Key      Value      Key

   1           9             2            8            3

   6           4             7            3            8

   (           22            )            23          

   E          12           R           11           T

   L          36           0            38           P

 ESC       31           A           46           S

   G         42            J            51           K

   +          29        NLN         30           Z

   V         57           B            56           N

               55           /             47         VTX

  SPC     15         CRL          2          CRU  

 Value     Key      Value      Key     Value

   7            4             6            5           5     

  19           9            20           0          21

  24           Q           14          W         13

  10           Y           37           U         35

  26           =           27            -          28

  45           D           44            F         43

  62           L           39             ;          40

  60           X           59            C         58

  52           M          53             ,          54

  25         HOM      63           INS       61

  50         CRD       34           CRR     18

ESC = Escape,   NLN=Newline

VTX = Videotext,   HOM = Home

INS = Insert,   SPC = Space

CRL = Cursor left

CRU = Cursor up

CRD = Cursor down

CRR = Cursor right

SHIFT adds 64 to value

CONTROL adds 128

GRAPHIC = adds 192


The other COP command which I have found useful is the one to rewrite the line display. Using this, the line display can be poked with values, and then updated by setting COPCTL to A0 (POKE 59,160).

 The following short program demonstrates that the line display can in fact display 256 different characters, not the 64 or so sated in the manual, the characters are displayed 16 at a time; pressing any key shows the next group.

10 CLOSE #1: OPEN #1,5

20 ba=62: FOR i=0 TO 255 STEP 16

30 FOR j=15 TO 0 STEP -1: POKE ba+j,i+15-j: NEXT j

40 POKE 59,160: GET #1,a: NEXT i

50 CLOSE #1: END

As you will notice, the top 128 characters are simply flashing versions of the bottom 128, and the ‘new’ characters are mostly pretty weird and wonderful …

I have written a game using the line display only using some of the new ‘graphics’ (a world first?), but a more practical use is to enable upper and lower case to be distinguished on the line display, by flashing lower case letters. The following subroutine does just that, displaying a$ in upper/lower case.

1000 IF LEN(a$)}16 THEN a$ = LEFT$(a$,16)

1010 FOR i=1 TO LEN(a$)

1020 a=ASC(MID$(a$,i))

1030 IF a}96 AND a{123 THEN a=a+96

1040 POKE 78-i,a

1050 NEXT i

1060 FOR i=i TO 16

1070 POKE 78-i,32

1080 NEXT i

1090 POKE 59,160


The loop from 1010 to 1050 puts the string into the buffer, converting any lower case letters to their flashing upper case equivalents. The loop from 1060 to 1080 clears the rest of the display, and 1090 makes the COP display the buffer.

By Stephen Burt

PCW 165

Digitized by Colin Appleby © 2008

Last Updated Sunday, 31 August 2008

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