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Molecular 18 Processor and Instruction Set
The Molecular word size is 18 bits, and the basic memory is 32K words.
The machine was generally known as the Molecular-18 for this reason.
The MSB (Bit 18) was a parity bit, so for all intents and purposes it was a
17 bit machine.
The minimal CPU contained the following registers:
PC - Program counter
MA - Memory address
REGA - Register A
REGB - Register B.
C - Carry Flag
GT - Greater Than Flag
Operation codes 2 through to 26 - Memory reference instructions
These instructions operated on either current page or zero page. All of these
instructions could operate indirectly.
Instruction words are organized as follows:
|17|16|15|14|13|12|11|10| 9| 8| 7| 6| 5| 4| 3| 2| 1|
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|OP-CODE |I |Z | ----- Memory Address ------
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I = Indirection Flag Z = Zero Page Flag
02
- JUMP Jump to
address 15 -
ADAC Add to Reg A with carry
03
- JSBR Jump to
subroutine 16 - ADBC Add to
Reg B with carry
04
- INSZ Increment and skip on zero 17 - SFAC Subtract from
Reg A with carry
05
- DESZ Decrement and skip on zero 20 - SFBC Subtract from
Reg B with carry
06
- ANDA Logical
AND
21 - LDA Load Reg A
07
- IORA Logical
OR
22 - LDB Load Reg B
10
- XORA Logical Exclusive
OR 23 - CMPA Compare with Reg A
11
- ADA Add to Reg
A
24 - CMPB Compare with Reg B
12
- ADB Add to Reg
B
25 - STA Store Reg A
13
- SFA Subtract from Reg
A 26 - STB Store Reg B
14
- SFB Subtract from Reg B
The CMPA and CMPB and other skip instructions conditionally skip the next instruction
in sequence. The INSZ and DESZ instructions were commonly used to increment or
decrement a loop counter and skip if
done, and it is also used as an general increment instruction, either followed
by a no-op or in contexts where it is known that the result will never be zero.
The JSBR instruction stores the return address in relative word zero of the
subroutine, with execution starting with relative word one. Subroutine
return is done with an indirect JUMP through the return address. Subroutines
commonly increment their return addresses to index through inline parameter
lists or to perform conditional skips over instructions following the
call. This is exactly the same as the PDP8
Operation code 0 - Register Instructions
No memory reference in this group, so they execute in one
machine cycle.
Micro-instructions could be combined, but only if from the
same group.
|17|16|15|14|13|12|11|10 | 9| 8| 7| 6| 5| 4| 3| 2| 1|
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|Opcode 0 |Mode |A/B| --- Micro Instruction -- |
| 12| 11|10 | 9 | 8 | 7 | 6
| 5 | 4 | 3 | 2 | 1
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|-----------------------------------------------------------------|
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| 0 | 1 |A/B|Clear|Left/
|Shift|Ro'te|With|Dec |Inc |Skip |Skip |
| | |1/0|Carry|Right|
| |carry|
| |B16=0|B1=0 |
|-----------------------------------------------------------------|
| 0 | 1 |A/B|Clear|One's|Clear|Comp |Skip |Swap |Clear|Comp |Read |
| | |1/0|Reg |Comp |Carry|Carry|
| |Sign |Sign |SWREG|
|-----------------------------------------------------------------|
| 0 | 1 |A/B|True |Left/
|Skip |Skip |Skip |Skip|Clear|Clear|One's|
| | |1/0|False|Right|-ve |not
0|carry|GT |GT |Reg |Comp |
|-----------------------------------------------------------------|
Mode
00 - Various instructions
0 NOP No
Operation
1 HALT Halt
Processor
2 MASK Interrupt
disable flags
3 ACKI Acknowledge
Interrupt
4 ION
Interrupts On
5 IOFF Interrupts
Off
6 SION Skip if
Interrupts On
7 SIOFF Skip if interrupts
Off
10 SMOF Skip if Mains Fail
interrupt
11 SMON Skip if mains on
interrupt
12 PRTY Skip if partiry
interrupt
13 PRCT Skip if memory
protect interrupt
14 BNDY Skip if memory
boundary interrupt
15 MASW Skip if MA=SR(Switch
Reg)
16 CONT Skip if Continuous
Interrupt switch on
17 RSTN I/O Reset
Mode
01 - Shift and Rotate (Applies to Reg B also if bit 10 not set)
2400 CLC Clear Carry (Wrong
group?)
3001 ALSB Skip if LSB of Reg A set
3002 AMSB Skip if MSB of Reg A set
3004 INCA Increment Reg A
3010 DECA Decrement Reg A
3300 LSA Left Shift Reg A (into
carry)
3100 RSA Right shift Reg A
3040 RRA Rotate right Reg A
3260 LRAC Left rotate Reg A with carry
3060 RRAC Right rotate Reg A with
carry
Mode 10 -
Clear and Complement (Applies to Reg B alsi if bit 10 not set)
5400 CLA Clear Reg A
5200 CPLA Complement Reg A
4100 CLC Clear carry flag
4040 CMPC Complement carry flag
4020 SKIP Skip next instruction
5010 SWPA Swop top and bottom half of
Reg A
5004 CLSA Clear sign (Bit 17) of Reg A
5002 CPSA Complement sign bit if Reg A
5001 ESRA Enter Switches in to Reg A
Mode 11 -
Alter/Skip
7200 APOS 2 Skip if Reg A
+ve (i.e. Bit 17 not set)
7100 A=0 2 Skip if
Reg A is zero
7002 CLA - Clear Reg
A
7001 CPLA - Complement Reg
A (NOT twos complement)
6040 SKNC 2 Skip if carry
not set
6010 SNGT 2 Skip if not
greater than set
6004 CLGT - Clear greater
than flag
7600 ANEG 1 Skip if Reg A
is -ve (i.e.Bit 17 not set)
7500 AN0 1 Skip if
Reg A is not zero
6440 SK=C 1 Skip if carry
flag set
6410 S=GT 1 Skip if
greater than flag set
Instructions
in the same group may not be combined.
Operation code 1 - Input / Output instructions
| 12 | 11 | 10 | 9 | 8 |
7 | 6 | 5 | 4 | 3 |
2 | 1 |
|------------------------------------------------------------------------|
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|A/B | Function | -- Mode -- |
------- Device Address ------- |
|------------------------------------------------------------------------|
|1/0 | 0 | 0 |
|Device Reg
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| |No Operation | |1, 2 or 3
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|------------------------------------------------------------------------|
| | 0 | 1
| |Device Reg
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| |Set Busy / | |1, 2, or
3
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| |Clear Done |
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|------------------------------------------------------------------------|
| | 1 | 0
| |Device Reg
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| |Clear Busy/ | |1, 2, or 3
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| |Clear Done |
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|------------------------------------------------------------------------|
| | 1 | 1 |Read/|Device
Reg | Read or Write transfer to device |
| |Input/Output |Write|1, 2, or 3 | register. Action
depends on the |
| |Pulse
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| | actual
device
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|------------------------------------------------------------------------|
| | 0 | 0 | 1
| 1 | 1
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| |Skip if Busy | Skip
Mode | No data transfer
during |
| | 0 | 1 | 1
| 1 | 1 | skip mode
instructions. |
| |Skip not Busy| Skip
Mode
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| | 1 | 0 | 1
| 1 | 1
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| |Skip if Done | Skip
Mode
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| | 1 | 1 | 1
| 1 | 1
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| |Skip not Done| Skip
Mode
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|------------------------------------------------------------------------|
Disk drives were assigned device codes from 70 to
77. Terminals and Printers were assigned device codes from 47 up to
67.
Although a console as such wasn't needed, a terminal
attached to device code 47 was necessary
Each attached device, in fact the device driver card,
had up to three registers. The actual function of the registers
depended on the attached device.
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