The structure of the instructions in the YASEP architecture

YASEP uses a 16-bit fixed-format instruction word with an optional 16-bit immediate field or a 16-bit extended word. This instruction format is identical for both YASEP16 and YASEP32.

Each instruction can contain :

2 flag bits (16/32 bit flag and immediate flag)
an opcode (6 bits)
up to 4 register addresses (4 bits each)
a 4-bit or a 16-bit sign-extended immediate value
a condition code (3 bits)
2 auto-update flags (2*2 bits)

These fields and flags are not all used at the same time. Some combinations are impossible by construction, some others don't make sense, others can be hidden by the assembler. However, a wide variety of instructions (an opcode followed by register names, immediate data or other flags) can be written by the user. This page explains what are the "instruction forms", how they are designed and when they are used.

Summary

An "instruction form" is named after the way it is written in assembly language. It is a sequence of letters that describe each used field :

R for a register
i for a short immediate number (4 bits)
I for a long immediate value (16 bits)

The YASEP allows only a limited number of forms. The two least significant bits of each instruction determine how to interpret the operands and other fields. The 2 bits create 4 combinations :

[00] Short form (16 bits) with 2 registers (RR)
[10] Short form (16 bits) with 1 register and one signed 4-bit immediate value (iR)
[01] Long form (32 bits) with 2 registers and one signed 16-bit immediate value (IRR)
[11] Extended long form (32 bits) with 3 registers (RRR), conditions and other features.

The extended form can also use one of the register fields as a signed 4-bit immediate value (iRR), so there are 5 available forms. However, the assembler can make more forms available to the programmer by using combinations with unused fields.

Where do results go ?

The YASEP instructions write the result of an operation (when any) to a register whose address is given by the si4, snd or dst3 fields, depending on the instruction form :

When the instruction is short (RR, iR etc.) then the result is written to snd.
When the instruction's form is "long immediate" (IRR or IR) then the result is written to si4.
When the instruction's form is "long extended" (RRR, iRR etc.) then the result is written to dst3.

It is justified to say that it is a bit complex and may make architectural development difficult in the far future. However, this is required to keep the instructions compact. Furthermore, in the datapath, what really counts is the access time of the operands, not the destination address which can be computed at the same time as the ALU (a good dozen of logic levels)

The "short forms"

Let's start with the most simple instruction forms. The short instructions have their LSB cleared and are 16-bit long. This is enough for another flag, a 6-bit opcode and two 4-bit fields.

The first 4-bit field is always a register address, named snd because it is used as a source, can undergo negation of all the bits (for substractions and boolean operations) and it is generally used as destination.
The second 4-bit field is interpreted as a sign-extended immediate value (from -8 to 7) when the 2nd LSB is set to 1, or as a register address when the 2nd LSB is 0. It is called si4 because it is used as source or immediate (4 bits) operand.

Note : before 20090730, these two fields were called src1 et src2 and were too confusing... They are renamed si4 and snd since 20090730 to avoid more mistakes.

"Short register form" : Two register addresses

ADD R2, R1 ; R1 <= R1 + R2

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
si4				snd				opcode						Reg	short
R2				R1				ADD						0	0

"Short immediate form" : one immediate and one register address

ADD -3, R1 ; R1 <= R1 + -3

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
si4				snd				opcode						Imm4	court
-3				R1				ADD						1	0

In practice, these 2 combinations can be used in 5 ways in assembly language, since any of these fields can be more or less useful depending on the kind of operation :

Form "RR" : Register to Register

 ADD R1 R2   ; R2 <- R1+R2

This is the most common form, where si4 and snd are both register operands, and the result goes to snd.

Form "iR" : short immediate to Register

 ADD 2 R3   ; R3 <- 2+R3

Another common form where si4 is used as an immediate value, so snd is used as both source and operand.

Form "R" : Register

 NEG R1   ; internally : R1 <- 0 - R1

It's also a short way to write that both si4 and si4 fields contain the same register address. The assembler will correctly fill the operand fields if the form is correct for the opcode.

Form "i" : short Immediate

 CRIT 3   ; disable the interrupts during the 3 following instructions

Not used very often but sometimes necessary. si4 is used as an immediate value and snd is ignored.

Form "ALONE" : no operand

 NOP   ; Nop operand needed, all fields ignored.

This is an extreme case where no operand is needed : si4 and snd are ignored.

The "long forms"

YASEP interprets the instruction as 32-bit long when the LSB is set to 1. The 16 lower bits are exactly the same as the short forms. The second LSB selects how to interpret the higher half-word :

When it is cleared, the additional field is a 16-bit sign-extended immediate value (ranging from -32768 to 32767) that is used as a source for the operation.
When it is set, it is an extended field that provides more addresses and functions.

"Long immediate form" : Two register addresses and one long immediate value

ADD R2, 1234h, R1 ; R1 <= 1234 + R2

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Imm16																si4				snd				opcode						imm16	long
1234h																R2				R1				ADD						0	1

Form "IRR" : long Immediate and Register to Register

 ADD 1234h R2 R1   ; R1 <- 1234h + R2

This is another common form where the operands are snd (a register field) and the 16-bit immediate field. The destination register is given by si4.

Form "IR" : long Immediate to Register

 GET 1234h R1   ; R1 <- SR[1234h]

This form is used for the GET or MOV instructions. si4 is ignored because only snd makes sense for the operation. The assembler also detects when the immediate address is larger than 4 bits and issues a long or a short form.

This is also used (in conjunction with the ALIAS_IRR flag) by the ROP2 group and the ADD/SUB instructions : this is similar to FORM_iR but with a 16-bit immediate operand. si4 and snd have the same value.

Form "RI" : Register to Immediate

 PUT R1 1234h   ; SR[1234h] <- R1

This form is currently only used for the PUT instruction. It is the same as FORM_IR but the order of the operands is reversed at the assembly language level to obey the rule #1 : "the destination is written at the end of the instruction".

The "extended forms"

The extended forms reuse the short form's structure and add 16 bits for several additional features :

A dedicated destination register field dst3 (bits 24 to 27) that overrides the short form's destination (snd becomes only a source)
An auxiliary Imm4 flag
A source register used for comparison (src4 at bits 28 to 31)
A condition selection field (3 bits)
Some bits remain for pointer updates.

These fields are located so that the other similar fields are 16 bits away (this simplifies the decoding logic).

"Extended Register form" : three register addressess

ADD R1, R2, R3 ; R3 <= R1 + R2

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
				dst3										Reg		si4				snd				opcode						Ext	long
				R3										0		R2				R1				ADD						1	1

Form "RRR" : Register and Register to Register

 ADD R1 R2 R3 ; R3 <- R1+R2

si4 (R1) and snd (R2) are both register operands, and the result goes to dst3 (R3).

"Extended Immediate form" : two register addressess and one 4-bit immediate field

ADD 5, R1, R3 ; R3 <= R1 + 5

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
				dst3										Reg		si4				snd				opcode						Ext	long
				R3										1		5				R1				ADD						1	1

Form "iRR" : imm4 and Register to Register

 ADD 5 R1 R3 ; R3 <- R1+5

The 4-bit immediate field si4 (5) and snd (R1) are the two operands, and the result goes to dst3 (R3).