COP8

Last updated January 04, 2024

The National Semiconductor COP8 is an 8-bit CISC core microcontroller. COP8 is an enhancement to the earlier COP400 4-bit microcontroller family. COP8 main features are:

Registers and memory map

The COP8 uses separate instruction and data spaces (Harvard architecture).^[1]^: 2-1^[2]^: 2-4 Instruction address space is 15-bit (32 KiB maximum), while data addresses are 8-bit (256 bytes maximum, extended via bank-switching).

To allow software bugs to be caught, all invalid instruction addresses read as zero, which is a trap instruction. Invalid RAM above the stack reads as all-ones, which is an invalid address.

The CPU has an 8-bit accumulator and 15-bit program counter. 16 additional 8-bit registers (R0–R15) and an 8-bit program status word are memory mapped. There are special instructions to access them, but general RAM access instructions may also be used.

The memory map is divided into half RAM and half control registers as follows:

COP8 data address space
Addresses	Use
0x00–6F	General purpose RAM, used for stack
0x70–7F	Unused, reads as all-ones (0xFF) to trap stack underflows
0x80–8F	Unused, reads undefined
0x90–BF	Additional peripheral control registers
0xC0–CF	Peripheral control registers.
0xD0–DF	General purpose I/O ports L, G, I, C and D
0xE0–E8	Reserved
0xE9	Microwire shift register
0xEA–ED	Timer 1 registers
0xEE	CNTRL register, control bits for Microwire & Timer 1
0xEF	PSW, CPU program status word
0xF0–FB	R0–R11, general purpose registers (additional RAM)
0xFC	R12, a.k.a. X, secondary indirect pointer register
0xFD	R13, a.k.a. SP, stack pointer register
0xFE	R14, a.k.a. B, primary indirect pointer register
0xFF	R15, a.k.a. S, data segment extension register

If RAM is not banked, then R15 (S) is just another general-purpose register. If RAM is banked, then the low half of the data address space (addresses 0x00–7F) is directed to a RAM bank selected by S. The special purpose registers in the high half of the data address space are always visible. The data registers at 0xFx can be used to copy data between banks.

RAM banks other than bank 0 have all 128 bytes available. The stack (addressed via the stack pointer) is always on bank 0, no matter how the S register is set.

Control transfers

In addition to 3-byte JMP and JSR instructions which can address the entire address space, 2-byte versions of these instructions can jump within a 4K page. The instruction specifies the low 12 bits, and the high 3 bits of the PC are preserved. (These are intended primarily for models with up to 4K of ROM.)

There are also jump indirect and load accumulator indirect instructions which use the accumulator contents as the low 8 bits of an address; the high 7 bits of the current PC are preserved.

For short-distance branches, there are 63 1-byte instructions which perform PC-relative branches from PC−32 to PC+31. This is a 15-bit addition, and no page boundary requirements apply.

Conditional branches per se do not exist, nor does the processor provide the traditional ZCVN status flags, although the program status word contains carry and half-carry flags for multi-byte arithmetic. Rather, there are a number of compare-and-skip instructions. For example, IFEQ compares its two operands, and skips the following instruction if they are unequal. Any instruction may be skipped; it is not limited to branches.

A feature unique to the COP8 architecture is the IFBNE instruction. This compares the low 4 bits of the B (memory pointer) register with a 4-bit immediate constant, and can be used to loop until B has reaches the end of a small (up to 16 byte) buffer.

An interesting extension of this mechanism is the RETSK return-and-skip instruction, which lets any subroutine call conditionally skip the following instruction. This provides a very compact way to return a boolean value from a subroutine.

Instruction set

COP8 operands are listed in destination, source order. Most instructions have the accumulator A as one of the operands. The other operand is generally chosen from an 8-bit immediate value, an 8-bit RAM address, or [B], the RAM address selected by the B register. Some instructions also support RAM addressing by the X register ([X]), and post-inc/decrement variants ([B+], [B−], [X+], [X−]).

Indirect addressing via B is particularly fast, and can be done in the same cycle that the instruction is executed.

On the other hand, absolute RAM addressing is not directly encoded in most cases. Rather, a special "direct addressing" prefix opcode, followed by a 1-byte address, may precede any instruction with a [B] operand, and changes it to a memory direct operand. This adds two bytes and three cycles to the instruction. (Conditional-skip instructions skip the prefix and following instruction as a pair.)

All "move" instructions are called LD (load) even if the destination is a memory address. Unusually, there are no LD instructions with the accumulator as a source; stores are done with the X instruction which exchanges the accumulator with the memory operand, storing A and loading the previous memory contents. (This takes no additional time; X A,[B] is a one-cycle instruction.)

There are instructions to fetch from tables in ROM. These combine the high 7 bits of the program counter (PCU) with the accumulator, fetch a byte from that address, and place it in the accumulator (LAID instruction) or the low 8 bits of the program counter PCL (JID instruction). Because the next instruction executed must be in the same 256-byte page of ROM as the table itself, a 256-entry table is not possible.

COP8 family instruction set^[1]^[3]^[2]^[4]
Opcode								Operands		Mnemonic	Cycles	Description
7	6	5	4	3	2	1	0	b2	b3	Mnemonic	Cycles	Description
0	0	0	0	0	0	0	0	—	—	INTR	7	Software interrupt (push PC, PC ← 0x00ff)
0	0	0	offset					—	—	JP +disp5	3	PC ← PC + offset; jump 1–31 bytes forward (offset=0 reserved)
0	0	1	0	addrhi				addrlo	—	JMP addr12	3	PC[11:0] ← address. Top 3 bits of PC preserved.
0	0	1	1	addrhi				addrlo	—	JSR addr12	5	Jump to subroutine: push PC, proceed as JMP.
0	1	0	0	k				—	—	IFBNE #imm4	1	Execute next instruction if (B & 15) ≠ k; skip if (B & 15) = k.
0	1	0	1	k				—	—	LD B,#imm4	1	B ← 15 − k (zero-extended)

0	1	1	0	0	0	0	0	k	—	ANDSZ A,#imm8^*	2	Skip if A & k = 0 (=IFBIT #bit,A)
0	1	1	0	0	0	0	1	addrlo	—	JSRB addr8^†	5	Push PC, jump to boot ROM subroutine at address^[5]
0	1	1	0	0	0	1	–	—	—	(reserved for boot ROM^†)^[5]
0	1	1	0	0	1	0	0	—	—	CLR A	1	A ← 0
0	1	1	0	0	1	0	1	—	—	SWAP A	1	A ← A<<4 \| A>>4; swap nibbles
0	1	1	0	0	1	1	0	—	—	DCOR A	1	Decimal correct after BCD addition
0	1	1	0	0	1	1	1	—	—	PUSH A^*	3	[SP] ← A, SP ← SP−1
0	1	1	0	1	bit			—	—	RBIT #bit,[B]	1	Reset (clear to 0) given bit of RAM

0	1	1	1	0	bit			—	—	IFBIT #bit,[B]	1	Test given bit of RAM, skip if zero
0	1	1	1	1	bit			—	—	SBIT #bit,[B]	1	Set (to 1) given bit of RAM

1	0	0	m	0	opcode			operand	—	ALU operations, A ← A op operand
1	0	0	0	0	opcode			—	—	OP A,[B]	1	ALU operation with A and [B] (with [address] using DIR prefix)
1	0	0	1	0	opcode			k	—	OP A,#imm8	2	ALU operation with A and immediate k
1	0	0	m	0	0	0	0	operand	—	ADC A,operand		C,A ← A + operand + C; add with carry
1	0	0	m	0	0	0	1	operand	—	SUBC A,operand		C,A ← A + ~operand + C (A − operand − ~C)
1	0	0	m	0	0	1	0	operand	—	IFEQ A,operand		Skip if A ≠ operand
1	0	0	m	0	0	1	1	operand	—	IFGT A,operand		Skip if A ≤ operand
1	0	0	m	0	1	0	0	operand	—	ADD A,operand		A ← A + operand (carry unchanged!)
1	0	0	m	0	1	0	1	operand	—	AND A,operand		A ← A & operand
1	0	0	m	0	1	1	0	operand	—	XOR A,operand		A ← A ^ operand
1	0	0	m	0	1	1	1	operand	—	OR A,operand		A ← A \|operand

1	0	0	0	1	0	0	0	—	—	IFC	1	Skip if carry clear
1	0	0	0	1	0	0	1	—	—	IFNC	1	Skip if carry set
1	0	0	0	1	0	1	0	—	—	INC A	1	A ← A + 1 (carry unchanged)
1	0	0	0	1	0	1	1	—	—	DEC A	1	A ← A − 1 (carry unchanged)
1	0	0	0	1	1	0	0	—	—	POP A^*	3	SP ← SP+1, A ← [SP]
1	0	0	0	1	1	0	1	—	—	RETSK	5	Pop PC, skip one instruction
1	0	0	0	1	1	1	0	—	—	RET	5	Pop PC high, pop PC low
1	0	0	0	1	1	1	1	—	—	RETI	5	Return and enable interrupts

1	0	0	1	1	0	0	0	k	—	LD A,#imm8	2	A ← k
1	0	0	1	1	0	0	1	k	—	IFNE A,#imm8^*	2	Skip if A = k
1	0	0	1	1	0	1	0	k	—	LD [B+],#imm8	3	[B] ← k, B ← B + 1
1	0	0	1	1	0	1	1	k	—	LD [B−],#imm8	3	[B] ← k, B ← B − 1
1	0	0	1	1	1	0	0	address	—	X A,addr8	3	A ↔ [address], exchange
1	0	0	1	1	1	0	1	address	—	LD A,addr8	3	A ← [address]
1	0	0	1	1	1	1	0	k	—	LD [B],#imm8	2	[B] ← k
1	0	0	1	1	1	1	1	k	—	LD B,#imm8^*	2	B ← k (one cycle faster than LD R14,#k)

1	0	1	0	0	0	0	0	—	—	RC	1	C ← 0; reset carry to 0
1	0	1	0	0	0	0	1	—	—	SC	1	C ← 1; set carry to 1
1	0	1	0	0	0	1	0	—	—	X A,[B+]	2	A ↔ [B], B ← B+1
1	0	1	0	0	0	1	1	—	—	X A,[B−]	2	A ↔ [B], B ← B−1
1	0	1	0	0	1	0	0	—	—	LAID	3	A ← ROM[PCU:A]; load from ROM
1	0	1	0	0	1	0	1	—	—	JID	3	PCL ← ROM[PCU:A]; jump via ROM table
1	0	1	0	0	1	1	0	—	—	X A,[B]	1	A ↔ [B]
1	0	1	0	0	1	1	1	—	—	(reserved)
1	0	1	0	1	0	0	0	—	—	RLC A^*	1	C,A ← A,C; rotate left through carry (=ADC A,A)
1	0	1	0	1	0	0	1	address	k	IFEQ addr8,#imm8^*	3	Skip if [address] ≠ k
1	0	1	0	1	0	1	0	—	—	LD A,[B+]	2	A ← [B], B ← B+1
1	0	1	0	1	0	1	1	—	—	LD A,[B−]	2	A ← [B], B ← B−1
1	0	1	0	1	1	0	0	addrhi	addrlo	JMPL addr15	4	PC ← address
1	0	1	0	1	1	0	1	addrhi	addrlo	JSRL addr15	5	Push PC, PC ← address
1	0	1	0	1	1	1	0	—	—	LD A,[B]	1	A ← [B]
1	0	1	0	1	1	1	1	—	—	(reserved)

1	0	1	1	0	0	0	0	—	—	RRC A	1	A,C ← C,A; rotate right through carry
1	0	1	1	0	0	0	1	—	—	(reserved)
1	0	1	1	0	0	1	0	—	—	X A,[X+]	3	A ↔ [X], X ← X+1
1	0	1	1	0	0	1	1	—	—	X A,[X−]	3	A ↔ [X], X ← X−1
1	0	1	1	0	1	0	0	—	—	VIS^*	5	PC ← ROM[vector table]; Vector Interrupt Select
1	0	1	1	0	1	0	1	—	—	RPND^*	1	Reset pending interrupt flag
1	0	1	1	0	1	1	0	—	—	X A,[X]	3	A ↔ [X]
1	0	1	1	0	1	1	1	—	—	(reserved)
1	0	1	1	1	0	0	0	—	—	NOP	1	No operation
1	0	1	1	1	0	0	1	—	—	IFNE A,[B]^*	1	Skip if A = [B]
1	0	1	1	1	0	1	0	—	—	LD A,[X+]	3	A ← [X], X ← X+1
1	0	1	1	1	0	1	1	—	—	LD A,[X−]	3	A ← [X], X ← X−1
1	0	1	1	1	1	0	0	address	k	LD addr8,#imm8	3	[address] ← k
1	0	1	1	1	1	0	1	address	—	DIR addr8	3	Change next instruction's operand from [B] to [address]
1	0	1	1	1	1	1	0	—	—	LD A,[X]	3	A ← [X]
1	0	1	1	1	1	1	1	—	—	(reserved)

1	1	0	0	register				—	—	DRSZ register	3	register ← register − 1, skip if result is zero
1	1	0	1	register				k	—	LD register,#imm8	3	register ← k (=LD 0xf0+register,#k, one byte shorter)
1	1	1	offset					—	—	JP −disp5	3	PC ← PC − 32 + offset; jump 1–32 bytes backward
7	6	5	4	3	2	1	0	b2	b3	Mnemonic	Cycles	Description

*: Only on "feature family" (COP888/COP8SA) cores; not present on "basic family" (COP800) cores.
†: Only on "flash family" (COP8TA/COP8C) models with boot ROM for in-system programming

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References

1 2 COP8 Basic Family User's Manual (PDF). Revision 002. National Semiconductor. June 1996. Literature Number 620895-002. Retrieved 2021-01-02.
1 2 Aleaf, Abdul (July 1996). "Comparison of COP878x to the Enhanced COP8SAx7 Family - Hardware/Software Considerations" (PDF). National Semiconductor. Application Note 1043.
↑ COP8 Feature Family User's Manual. Revision 005. National Semiconductor. March 1999. Literature Number 620897-005. Extracted from zipped ISO image 530094-003_COP8_Tools_Docs_Aug1999.zip, retrieved 2020-01-07.
↑ "COP8SAx Designer's Guide" (PDF). National Semiconductor. January 1997. Literature Number 620894-001.
1 2 "COP8SBR9/COP8SCR9/COP8SDR98-Bit CMOS Flash Based Microcontroller with 32k Memory, Virtual EEPROM and Brownout" (PDF) (data sheet). National Semiconductor. April 2002. Retrieved 2021-01-06.

External links

National Semiconductor Embedded Microcontrollers (CR16 and COP8)
microcontroller-faq/COP8, 1995
"National Semiconductor COP8". EDN’s 25th Annual Microprocessor/Microcontroller Directory. EDN . 24 September 1998.

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[basic-1] 1 2 COP8 Basic Family User's Manual (PDF). Revision 002. National Semiconductor. June 1996. Literature Number 620895-002. Retrieved 2021-01-02.

[comparison-2] 1 2 Aleaf, Abdul (July 1996). "Comparison of COP878x to the Enhanced COP8SAx7 Family - Hardware/Software Considerations" (PDF). National Semiconductor. Application Note 1043.

[feature-3] COP8 Feature Family User's Manual. Revision 005. National Semiconductor. March 1999. Literature Number 620897-005. Extracted from zipped ISO image 530094-003_COP8_Tools_Docs_Aug1999.zip, retrieved 2020-01-07.

[4] "COP8SAx Designer's Guide" (PDF). National Semiconductor. January 1997. Literature Number 620894-001.

[cop8s-5] 1 2 "COP8SBR9/COP8SCR9/COP8SDR98-Bit CMOS Flash Based Microcontroller with 32k Memory, Virtual EEPROM and Brownout" (PDF) (data sheet). National Semiconductor. April 2002. Retrieved 2021-01-06.

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