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<h2>6. Gate Microcode and Macrocode Compiler</h2>

Gmac is a simple microcode and macrocode compiler which can be used to
create memory files for TkGate.  You can write declarations to define
your own microcode and macrocode.  The current version of gmac is
fairly simple; no attempt is made to handle arbitrary instruction
sets with complex addressing modes, but it should be sufficient to
create simple micro- and macro-programs.

To compile a micro/macro program description, use the command:

<pre>
gmac infile.gm -o outfile.mem -l outfile.map
</pre>

This will read the description in "infile.gm", and generate the memory
file "outfile.mem" and and the optional map file "outfile.map".  The
memory file will contain the appropriate "memory" keywords to load
multiple memories.  The map file contains the list of symbols defined
in the input file in a human-readable form.
<p>
Gmac input files consist of a sequence of declarations.  Whitespace
and newlines are ignored, and comments are indicated by a '//'.  A
complete example of a Gmac input file is given in <a href="menagerie.html">Appendix E</a>.

<a name="banks"></a>
<h3>6.1 Defining Memory Banks</h3>

The memories which will be generated are declared with "bank"
statements.  A bank statement has the form:

<br><br>
<i>type</i> bank [<i>msb</i>:<i>lsb</i>] <i>name</i> ?[<i>first</i>:<i>last</i>]?;
<br><br>

The <i>type</i> parameter is one of "microcode", "macrocode" or "map"
and specifies what type of memory this is a bank for.  Microcode and
macrocode banks specify the memories to be used for the micro- and
macro-programs respectively.  Map banks are typically used to look up
the microaddress for implementing a macro-instruction.  Each of the
memory types is optional and can be omitted.
<p>
The bit range [<i>msb</i>:<i>lsb</i>] specifies the portion of the
word which will be stored at an address in the memory <i>name</i>.
The specified memory should be the full path name of a memory in the
circuit file.  A word can be broken across multiple memories by using
multiple bank statements.  A single bank is limited to 32 bits, so
memories with very long words, such as a micro-program will probably
have to be broken into multiple memories.  An optional address
specifier after the memory name specifies the range of address which
this memory will hold.  If no address range is specified, this memory
will be used for all words.

As an example, consider the set of bank declarations:
<pre>
microcode bank[31:0] iunit.m1;
microcode bank[63:32] iunit.m2;
map bank[7:0] iunit.map;
macrocode bank[7:0] memory.m1;
</pre>

This indicates that the mirco-program will be consist of 64-bit
instructions with the low half of the instructions being stored in
"iunit.m1" and the high half of the instructions being stored in
"iunit.m2".  The map memory contains 8-bit data and is stored in
"iunit.map".  The main macrocode memory has 8-bit words and is stored
in "memory.m1".

<a name="microinst"></a>
<h3>6.2 Defining the Micro-Instruction Set</h3>

Microcode instructions are defined using the 'field' declaration.  A
field declaration consists of the 'field' keyword followed by one or
more comma-separated declarators.  A simple declarator is simply a
name followed by a bit range in square brackets.  For example:

<pre>
field cin[18], idata[9:2];
</pre>

declares that bit 18 of the microinstruction has the name "cin", and
the bits 2 through 9 have the name "idata".  Single bit fields can
optionally be prepended with the "~" to indicate they are active low
signals.

A set of symbolic values can also be supplied for a field.  For
example:

<pre>
field mpcop[1:0]={
	 next=0,	// Increment mpc
	 reinit=1,	// Restart CPU
	 jmap=2,	// Jump from map value
	 jump=3		// Jump from condition
};
</pre>

declares a 2-bit field "mpcop" with a set of symbolic names for field
values.  Hexadecimal symbolic values can be specified by prepending them
with a "0x".

<a name="microprog"></a>
<h3>6.3 Writing the Micro-Program</h3>

A block of microcode is defined using the "begin microcode @"
specifier.  A start address must be specified after the declaration.
The end of the block is indicated with the "end" keyword.  The
micro-program defined in the block will be stored starting at the
specified address.  Microinstructions consists of a list of fields
(defined by the 'field' keyword described above), with an optional
value specification.  The microinstruction is ended by a ";".  Micro
instructions can be labeled by specifying an identifier followed by a
colon.
<p>
For each single-bit field, the presence of a field name in a
microinstruction indicates that that bit should be asserted.  For
normal fields, this means setting the bit to 1 in the
microinstruction.  For active low fields (indicated with a "~" symbol
in the field declaration) this means setting the bit to 0.  The bit
corresponding to any active low fields not specified in an instruction
will be set to 1.

For multi-bit fields, a value should be specified after an '=' symbol.
The value can be either a fixed constant, a symbolic value defined for
the field, or a label which will be resolved to the appropriate
microinstruction address when the specification is compiled.

<p>
For example, consider the microcode fragment:
<pre>
begin microcode @ 0
start:	clq;								// Q <- 0
	idata=0x1 ALU_AZERO ALU_OP=add bop=idata ldqh;			// Q.H <- 1
	ALU_AZERO ALU_OP=add bop=qreg dout ldpc;			// PC <- Q
	mpcop=jump mpccond=jmp mpcaddr=fetch;				// jump to 'fetch'
	mpcop=next;
end
</pre>
This specifies that this fragment will begin at address 0.  The first
mirco instruction is labeled 'start' and simply asserts the signal 'clq'.
In this case, this clears the contents of the Q register.  The next
micro-instruction puts the value '1' on the field name "idata",
asserts the signal "ALU_AZERO", puts the symbolic value "add" on the
field "ALU_IP", the symbolic value "idata" on the field "bop" and
asserts the signal "ldqh".  This set of operations causes a 1 to be
stored in the high half of the Q register.  The subsequent instructions
load the program counter PC from the value stored in the Q register
(0x100), and jumps to the label 'fetch'.

<a name="macroinst"></a>
<h3>6.4 Defining the Macro-Instruction Set</h3>

The macro-instruction set is defined using "register", "operands" and
"op" declarations.  The "register" keyword is used to define the
register names used in the macrocode and assign them a numeric value.
<p>
For example:
<pre>
registers R0=0, R1=1, R2=2, R3=3, R4=4, R5=5, FP=6, SP=7;
</pre>
declares 8 registers name R0 through R5, FP and SP.  With numeric
values as specified in the declaration.  The numeric value here will
be used in building the macroinstruction using a register.

<h4> 6.4.1 Operand Declarations</h4>

The "operands" declaration specifies a set of operand types that an
instruction can have.  An operands declaration consists of the
"operand" keyword, a symbolic name, and a list of addressing modes.
Currently only four addressing modes are recognized: immediate,
register direct, register indirect, and indexed.  Since the syntax of
the operand declarations is rather complicated, it will be explained
with an example.  For instance, suppose we wish to define an 'add'
instruction which can use the following addressing mode combinations:

<pre>
   add	R1,#1234	// register direct, immediate
   add  R1,R2		// register direct, register direct
   add  R1,(R2)		// register direct, register indirect
   add  R1,#Array(R3)	// register direct, indexed
</pre>

We can define the addressing mode combinations with the declaration:
<a name="myopts"></a>
<pre>
operands myopts {
    %1,#2     = { +0[1:0]=0; +1[7:4]=%1; +1[3:0]=0; +2=#2[7:0]; +3=#2[15:8]; };
    %1,%2     = { +0[1:0]=1; +1[7:4]=%1; +1[3:0]=%2; };
    %1,(%2)   = { +0[1:0]=2; +1[7:4]=%1; +1[3:0]=0; +2=#2[7:0]; +3=#2[15:8]; };
    %1,#2(%3) = { +0[1:0]=3; +1[7:4]=%1; +1[3:0]=%3; +2=#2[7:0]; +3=#2[15:8]; };
};
</pre>
This block defines a set of 4 operand combinations for a two-operand
instruction.  Each operand combination is specified by a comma
separated list of addressing modes.  A '-' can be used in place of the
operand list to indicate instructions with no operands.  Registers are
indicated by a '%' followed by an id number, and numbers are indicated
by a '#' followed by an id number.  The statements in braces following
an address mode combination specify a sequence of assignments used to
build the macroinstruction.  Each assignment contains a word number
within the macroinstruction, an optional bit range, and a value to
store at the specified location.  If the value is a simple number, the
specified number will be stored.  If the value is prepended with a '%'
or '#', then the value is taken to be the id number for a register or
number specified in the instruction.  For example, on a machine with a
byte-addressable main memory, the previous operand declarations will
generate the bytes:

<pre>
   +-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+
   | undefined |1 1|   |0 0 0 1|0 1 0 1|  |0 0 1 1|1 0 0 0|  |0 0 0 1|0 0 1 0|
   +-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+  +-+-+-+-+-+-+-+-+
</pre>

for the instruction:

<pre>
   add  R1, #0x1234(R5)
</pre>

Since the addressing modes are "register" and "indexed", the last
entry in the list of operand combinations applies.  The "+0[1:0]=3"
statement sets the low two bits of the first byte to binary '11'.  The
"+1[7:4]=%1" statement sets the high half of the second bytes to the
code for the R1 register, and the "+1[3:0]=%3" statement sets the low
half of the second word to the code for the R5 register.  The
"+2=#2[7:0]" and "+3=#2[15:8]" statements store the index value '0x1234'
in the third and fourth bytes of the instruction low byte first.

<p>
One last type of memory assignment is the relative address assignment.
Relative addresses are indicated with the '@' operator.  The pair of
statements "+2=#2@[7:0]" and "+3=#2@[15:8]" will store the difference
of the operand with id 2 and the address of the first byte of the
instruction.  This feature can be used to define relative branching
instructions.  An optional value can be specified after the '@' as a
further fixed offset.

<h4> 6.4.2 Macro-Instruction Declarations</h4>

After defining a set of operand declarations, you must define the
macro-instruction set.  A macro-instruction declaration consists of
the 'op' keyword, an identifier, and a definition in braces.  The
definition can include memory assignment statements as described in
the section on operands above, 'map' statements and an operand
declaration.  For example:

<pre>
op add {
  map add_ri : 0x68;	// register direct, immediate
  map add_rr : 0x69;	// register direct, register direct
  map add_rd : 0x6a;	// register direct, register indirect
  map add_rx : 0x6b;	// register direct, indexed
 +0[7:0]=0x68;
  operands myopts;
};
</pre>
defines the 'add' instruction with the operand set defined by <a
href="#myopts">myopts</a>. The statement '+0[7:0]=0x68' defines the
base opcode for the instruction and the position to store it in the
instruction word.  The 'map' statements define map memory values that
can be used to map an opcode to a microinstruction address.  The left
side of the 'map' statement specifies a label in the microprogram.
The right side specifies an opcode value which maps to that address.
Since the low two bits of the first byte are used to indicate the
addressing mode, we need four map statements for each of the four
operand combinations.  The map declarations shown here will cause the
map memory to contain the addresses for the microinstructions add_ri,
add_rr, add_rd and add_ri at the addresses 0x68, 0x69, 0x6a and 0x6b
in the map memory.

<a name="macroprog"></a>
<h3>6.5 Writing the Macro-Program</h3>

Similar to the micro-program block, the macro-program block is started
by the "begin macrocode" specifier and ended by the 'end' specifier.
For example:

<pre>
  begin macrocode @ 0x100
</pre>

will start a block of macrocode at address 0x100.  Unlike other
portions of the gmac specification, newlines are significant in the
macrocode block and indicate the end of a macroinstruction.  A macro
program consists of a list of instructions and directives.  Any of the
macroinstructions defined can be used, and labels can be defined by
starting a line with an identifier followed by a ":".

<br><br>

<center><table border>
<tr>
<th>Directive</th>
<th>Description</th>
</tr>

<tr>
<td><i>label</i>: .symbol <i>value</i></td>
<td>Defines a label to be a specified value.</td>
</tr>

<tr>
<td>.short <i>value, ...<i></td>
<td>Inserts a set of two-byte values at successive memory locations.</td>
</tr>

<tr>
<td>.long <i>value, ...<i></td>
<td>Inserts a set of four-byte values at successive memory locations.</td>
</tr>

<tr>
<td>.byte <i>value, ...<i></td> <td>Inserts a set of byte values at
successive memory locations.  This directive can also take strings
denoted by double quotes with each byte of the string being stored at
successive locations.</td>
</tr>

<tr>
<td>.bss <i>value<i></td>

<td>Reserves <i>value</i> bytes of unallocated memory at the current
position.  This directive can be used to define uninitialized global
variables.</td>
</tr>


<tr>
<td>.proc <i>label</i></td>
<td>This directive can be used in conjunction with ".end" to define a
procedure.  The label specified will be assigned the current address,
but any labels defined between the ".proc" and the ".end" directives
will be treated as local labels.</td>
</tr>

<tr>
<td>.end</td>
<td>Ends a procedure definition.</td>
</tr>


</table></center>

<br><br>
Here is a short example of a macro program:

<pre>
begin macrocode @ 0x100
ttydata:	.symbol	0x11	// tty data in/out register
ttystatus:	.symbol	0x10	// tty status register
start:
	movw	SP, 0		// Initialize the stack pointer to 0
	movw	FP, 0		// Initialize the frame pointer to 0

	push	msg		// Push msg on the stack.
	call	print		// Call the procedure 'print'
	add	SP,2		// Reset the stack pointer.

sdone:	jmp	sdone		// Hang in an infinate loop.

////////////////////
//	print(s)	Print the string s
//
.proc print
	movw	R1, 4(FP)	// Store the argument value in R1
loop:	movb	R2, (R1)	// Store the byte addressed by R1 in R2
	jeq	done		// If the byte was a zero, jump to 'done'
	movb	(ttydata),R2	// Store the byte in the tty data register
	movb	(ttystatus),#1	// Write a 1 to the tty status register to output the byte
	add	R1, #1		// Add 1 to the R1 register
	jmp	loop		// Jump back to 'loop' to get the next byte.
done:	ret
.end

msg:	.byte	"Hello World.\n", 0
</pre>

Given the appropriate circuit, mircrocode and macrocode definitions,
this program will print the message "Hello World." on a tty and halt
by entering an infinite loop.


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