RRISC Assebmly - introduction
The RRISC CPU can address the following:
- the 16bit program counter
- 7 registers (A..G)
- 64k of RAM
- 64k port addresses
Basic commands
The RRISC CPU understands the following basic commands, that are all based on the read / write principle:
- LD register
- ST register
- JMP address
LD stands for load and is used for loading values into a register reg.
ST stands for store and is used for writing the value of a register to the RAM (or external ports).
Each of the above commands causes a transaction between elements of two groups: The first group consists of the 7 registers and the second group is is comprised of RAM, ports and instruction register 2 (‘operand’ of the instruction).
The following types of transactions are valid:
- register —> RAM
- register —> port
- register <— RAM
- register <— port
- register <— operand (instruction register 2)
JMP stans for jump and is used to continue program execution at a defined address. Strictly speaking, a jump is nothing else but an LD of the program counter - but because of its side effect, it is considered a distinct command.
LD and ST can be applied to 7 registers. The registers are named A, B, C, D, E, F, G.
Conditions
In addition, all commands can be executed under four different conditions:
- unconditional : instruction is always executed
- EQ (equal) : instruction is executed if the ALU’s EQ/zero flag is set
- GT (greater than) : instruction is executed if the ALU’s GT/carry flag is set
- LT (less than) : instruction is executed if the ALU’s LT flag is set
The ALU is the CPU’s arithmetic logical unit, and used to perform calculations and comparisons. The result of an ALU operation can be non-numerical, e.g. in a comparison operation. In that case, above-mentioned ALU flags are set instead.
Adressing modes
The RRISC CPU has the following addressing modes:
- immediate: loading a constant directly into a register or the program counter (jump)
- absolute : the source or destination is an address in memory
- external : the source or destination is a port address
- indirect : the source or destination address has a constant high byte, but the low byte will be fetched from RAM / port at the given address
Immediate addressing mode
This is the simplest of them all. A register is loaded with an immediate value, a constant specified by the programmer.
Example: LDA # $01 - loads the value $01 into register A
Absolute addressing mode
In absolute addressing mode, the operand is a 16-bit address to the RAM.
Examples:
LDA $1300 ; load byte from RAM at address $1300
STA $1301 ; store byte into RAM at address $1301
External addressing mode
In external addressing mode, the operand is a 16-bit address of an external port. External ports are used to communicate with periphery. In our case, the ALU is external periphery.
Examples:
IN A, $0000 ; read from port 0
OUT A, $0001 ; write to port 1
Indirect addressing mode
This mode is only applicable to JMP. It involves an indirection and is both interesting and also a bit more complicated. Indirect addressing can be used for both RAM and port access. It is best described by example:
Consider JMP $1300 - here the address is 1300 and can be split into a HIGH byte $13 and a LOW byte $00. The 16-bit address $1300 can be calculated from HIGH and LOW bytes as follows:
addr = HIGH * 256 + LOW
In indirect mode addressing, we write the instruction:
JMP $13[$00] ; HIGH[LOW]
… and calculate the address as follows:
temp_addr := $1300
LOW := byte at address $1300
HIGH:= $13
addr = HIGH * 256 +LOW
So first we look up the LOW byte of the address at the given address and then use the HIGH byte and the new LOW byte as address.
We can write the address calculation without using a temporary, using square brackets as ‘look into address’ indirection operator:
addr = HIGH * 256 + [HIGH * 256 + LOW]
This mode can be used for jump tables.
Assembler commands
Based on:
- read / write
- condition
- register / program counter
- addressing mode
the following assembler command set of the RRISC CPU is derived.
Note: the condition ‘unconditional’ is implicit. Unconditional commands do not have a condition clause.
Example:
lDA # $00 ; unconditional
LDA # $00 : EQ ; only if EQ flag is set
LD(reg) Addr : condition
Loads register reg with the RAM contents at address addr if the given condition is true.
Example:
LDA $1300 ; loads A with value at $1300
ST(reg) Addr : condition
Stores value of register reg into RAM at address addr if the given condition is true.
Example:
STA $1300 ; stores a into RAM at $1300
LD(reg) # val : condition
Loads the value val into register reg if the given condition is true.
Example:
LDA # $00 ; loads A with value $00
IN reg, port : condition
Loads the value at external port port into register reg if the given condition is true.
Examole:
IN A, $0003 ; read from port 3
OUT reg, port : condition
Writes the value of register reg to external port port if the given condition is true.
Example:
OUT A, $05 ; write to port 5
JMP addr : condition
Jump to address addr if the given condition is true.
Example:
JMP $C000 : EQ ; if equal, jump to $c000
JMP HI[LO] : condition
Jump to RAM address HI * 256 + [HI * 256 + LO] if the given condition is true.
Example:
JMP $C0[00] : EQ ; if equal, jump to $c000 + [$c0000]
JMPP HI[LO] : condition
Jump to address HI * 256 + ports[HI * 256 + LO] if the given condition is true. The difference to JMP is that here the LO byte is read from the specified port.
Example:
JMPP $C0[$00] : EQ ; if equal, jump to $c000 + port[$c000]
^ toc