Scott CPU assembler

A machine code assembler for the CPU designed in the book But How Do It Know? by J. Clark Scott.

Build

The assembler consists of a single .c file that can be built using:

make

from the projects root directory.

The assembler was built and has been tested using clang on Mac OS X but should compile using gcc on Linux just fine.

Run

To invoke the assembler, use:

./sca <file>

where <file> is the name of the assembly file you want to compile. For example to compile the multiply.asm example, issue:

./sca examples/multiply.asm

This will output a file with .bin replacing .asm extension in the same directory i.e. examples/multiply.bin. This is the binary output file. Note: this file extension replace will replace the extension after the first . found in the file or append .bin if no file extension is found e.g:

my_assembly_file.txt -> my_assembly_file.bin

my_assembly_file2 -> my_assembly_file2.bin

Instruction Set

Arithmetic and Logic Instructions

Opcode (binary)	Opcode (hex)	Mnemonic	Operands	Description	Operation	Flags	#Clocks
1000____	8_	ADD	RA, RB	Add the contents of RA to RB and store result in RB	RB ← RA + RB	C, A, E, Z	3
1001____	9_	SHR	RA, RB	Right shift the contents of RA and store result in RB	RB ← RA >> 1	C, A, E, Z	3
1010____	A_	SHL	RA, RB	Left shift the contents of RA and store result in RB	RB ← RA << 1	C, A, E, Z	3
1011____	B_	NOT	RA, RB	NOT RA and store the result in RB	RB ← ~RA	A, E, Z	3
1100____	C_	AND	RA, RB	AND RA and RB and store the result in RB	RB ← RA & RB	A, E, Z	3
1101____	D_	OR	RA, RB	OR RA and RB and store the result in RB	RB ← RA | RB	A, E, Z	3
1110____	E_	XOR	RA, RB	XOR RA and RB and store the result in RB	RB ← RA ^ RB	A, E, Z	3
1111____	F_	CMP	RA, RB	Compare RA and RB	RA - RB	A, E, Z	3

Load and Store Instructions

Opcode (binary)	Opcode (hex)	Mnemonic	Operands	Description	Operation	Flags	#Clocks
0000____	0_	LD	RA, RB	Load RB from RAM at RAM address in RA	RB ← [RA]	-	(2) 3
0001____	1_	ST	RA, RB	Store RB into RAM at RAM address in RA	[RA] ← RB	-	(2) 3

Data Instruction

Opcode (binary)	Opcode (hex)	Mnemonic	Operands	Description	Operation	Flags	#Clocks
001000__	2_	DATA	RB, K	Load byte constant, K into RB	RB ← K	-	3

Branch Instructions

Opcode (binary)	Opcode (hex)	Mnemonic	Operands	Description	Operation	Flags	#Clocks
001100__	3_	JMPR	RB	Jump to the RAM address stored in RB	IAR ← RB	-	(1) 3
01000000	40	JMP	K	Jump to the RAM address K	IAR ← K	-	(2) 3
01011000	58	JC	K	Jump to RAM address K if the 'carry' flag is set	If (C == 1) then IAR ← K	-	3
01010100	54	JA	K	Jump to RAM address K if the 'a is larger' flag is set	If (A == 1) then IAR ← K	-	3
01010010	52	JE	K	Jump to RAM address K if the 'equal' flag is set	If (E == 1) then IAR ← K	-	3
01010001	51	JZ	K	Jump to RAM address K if the 'zero' flag is set	If (Z == 1) then IAR ← K	-	3
01011100	5C	JCA	K	Jump to RAM address K if C or A are set	If (C == 1 or A ==1) then IAR ← K	-	3
01011010	5A	JCE	K	Jump to RAM address K if C or E are set	If (C == 1 or E ==1) then IAR ← K	-	3
01011001	59	JCZ	K	Jump to RAM address K if C or Z are set	If (C == 1 or Z ==1) then IAR ← K	-	3
01010110	56	JAE	K	Jump to RAM address K if A or E are set	If (A == 1 or E ==1) then IAR ← K	-	3
01010101	55	JAZ	K	Jump to RAM address K if A or Z are set	If (A == 1 or Z ==1) then IAR ← K	-	3
01010011	53	JEZ	K	Jump to RAM address K if E or Z are set	If (E == 1 or Z ==1) then IAR ← K	-	3
01011110	5E	JCAE	K	Jump to RAM address K if C or A or E are set	If (C == 1 or A ==1 or E == 1) then IAR ← K	-	3
01011101	5D	JCAZ	K	Jump to RAM address K if C or A or Z are set	If (C == 1 or A ==1 or Z == 1) then IAR ← K	-	3
01011011	5B	JCEZ	K	Jump to RAM address K if C or E or Z are set	If (C == 1 or Z ==1 or Z == 1) then IAR ← K	-	3
01010111	57	JAEZ	K	Jump to RAM address K if A or E or Z are set	If (A == 1 or E ==1 or Z == 1) then IAR ← K	-	3
01011111	5F	JCAEZ	K	Jump to RAM address K if C or A or E or Z are set	If (C == 1 or A ==1 or E == 1 or Z == 1) then IAR ← K	-	3

Clear Flags Instruction

Opcode (binary)	Opcode (hex)	Mnemonic	Operands	Description	Operation	Flags	#Clocks
01100000	60	CLF	-	Clear all flags	C = A = E = Z = 0	C, A, E, Z	(1) 3

IO Instructions

Opcode (binary)	Opcode (hex)	Mnemonic	Operands	Description	Operation	Flags	#Clocks
011100__	7_	IND	RB	Input IO data to RB	IO (data) ← RB	-	(1) 3
011101__	7_	INA	RB	Input IO address to RB	IO (address) ← RB	-	(1) 3
011110__	7_	OUTD	RB	Output RB to IO as data	RB ← IO (data)	-	(1) 3
011111__	7_	OUTA	RB	Output RB to IO as address	RB ← IO (address)	-	(1) 3

TODO

• -o output file cmd line option
• label implementation for branching rather than specifying absolute addresses
• org directive to add constants to label addresses