**Update Log**

2016/10/03 : Initial posting. Most of pipelining figured out. Working through DSP input and operation details. Have to think through data and return stack usage cases, decide if data register file should just be removed and replaced with indexed fetched from data stack.

2016/10/02 : Trying a different design path. Concerned that the core which enables easy factored code, and thus well compressed code in limited memory, is not having to optimize around a CPU pipeline from the perspective of a thread of execution. So in this try, I'm working through paper implementation of a core which round-robins through 4 threads for a 4 stage pipeline (talked about in this prior post). Maintaining variable bit-width address windows, and other things from prior post.

**Notes**

================ FORTH HYBRID ================ Dual stack machine with register file Core functional units, 16-bit x 8-entry return stack (1 port) 32-bit x 8-entry data stack (1 port) 32-bit x 8-entry register file (2 ports, port 0 for DSP input, port 1 for BRAM address) 32-bit x 1024-entry BRAM (2 ports, port 0 for instruction fetch, port 1 for data) =================== EXECUTION MODEL =================== 4 threads/core of execution running with guaranteed round-robin scheduling Instructions are VLIW style with a fixed logical ordered set of operations Order Operation ===== ========= 1st mux inputs for DSP ............ uses loads from prior instruction 2nd DSP execution ................. 3nd load/store to REG and BRAM .... can store DSP result 4th branch ........................ can branch to DSP result ===================== PHYSICAL PIPELINE ===================== Designing under the following constraints, Loads from RAMs are not used until next stage Each stage does only one LUT or ADD both of which can be vertically chained DSP is fully pipelined Outline, DSP DSP DSP DSP DSP DAT ADR BLK BLK Stage A B MUL C ADD P REG REG OUT RAM PC INS ===== === === === === === === === === === === === 0 lut lut @ 1 mul reg lut 2 add lut add 3 lut @! lut @! lut @ ----- --- --- --- --- --- --- --- --- --- --- --- DSP A B .... DSP input a and b arguments DSP MUL .... DSP mul stage DSP C ...... DSP input c argument DSP ADD .... DSP add/op stage DAT REG .... Register file data load/store ADR REG .... Register file address register to BRAM address translation BLK OUT .... BRAM data construct write value BLK RAM .... BRAM data load/store PC ......... Update program counter INS ........ Fetch next instruction ============================ CURRENT LUT BUDGET USAGE ============================ Budget is 400 LUTs/core, adding as design is roughed out, LUTs % usage ==== === ===== 32 8 register file (4x 8-LUT SLICEM 32-entry x 8-bit 2 port RAM) 16 4 data stack (2x 8-LUT SLICEM 32-entry x 16-bit 1 port RAM) 8 2 return stack ( 8-LUT SLICEM 32-entry x 16-bit 1 port RAM) ---- --- ----- 54 DSP b input DSP a input DSP c input 18 5 BRAM address generation 34 BRAM output generation 38 10 program counter ==== === ===== 200 total ======== TODO ======== Make sure to register all inputs required to generate a b and c =========================== DSP B INPUT =========================== Need to also unpack BRAM load options so this gets expensive Placement in pipeline, stage action ===== ====== 0 LUT DSP b input 1 2 register pre-translated address for stage 0 of next cycle 3 register pre-translated address for stage 0 of next cycle The b input expanded with unpack options, and control bits, fedcba9876543210 n LUT input count ================ = =============== <-----iiiiiiiiii i 1-bit tttttttttttttttt t 1-bit dddddddddddddddd d 1-bit ================ aaaaaaaaaaaaaaaa f 4-bits for MSB 8-bits of output bbbbbbbbbbbbbbbb 7-bits for 2nd LSB 4-bits of output cccccccccccccccc 15-bits for LSB 4-bits of output 00000000dddddddd 00000000eeeeeeee 00000000ffffffff 00000000gggggggg 000000000000hhhh 000000000000iiii 000000000000jjjj 000000000000kkkk 000000000000llll 000000000000mmmm 000000000000nnnn 000000000000oooo ================ xxxxxxxxxxxxxxxx needs 2-bit opcode control xxxxxxxxxxxxxxxx needs 2-bit MSB of pre-translate address xxxxxxxx........ needs 1-bit LSB of pre-translate address ........xxxx.... needs 2-bit LSB of pre-translate address ............xxxx needs 3-bit LSB of pre-translate address ================ xxxxxxxx........ 12:1 function (2 LUT/bit) x 8-bit = 16 LUT ........xxxx.... 16:1 function (4 LUT/bit) x 4-bit = 16 LUT ............xxxx 25:1 function (4 LUT/bit) x 4-bit = 16 LUT LUT area estimate, LUTs usage ==== ===== 48 generate b 6 to register 5-bits x 2 stages of pre-translate address (rounded up) ---- ----- 54 total =========================== DSP A INPUT =========================== Placement in pipeline, stage action ===== ====== 0 LUT DSP a input 1 2 3 LUT area estimate, LUTs usage ==== ===== ---- ----- total =========================== DSP C INPUT =========================== Placement in pipeline, stage action ===== ====== 0 LUT DSP c input 1 register c 2 3 LUT area estimate, LUTs usage ==== ===== ---- ----- total =========================== BRAM ADDRESS GENERATION =========================== Supports the feature of variable-bit width windows into the 4KB of ram Placement in pipeline, stage action ===== ====== 0 fetch base address from register file 1 optionally XOR immediate 2 translate into BRAM address 3 Implementation requires XOR control to be single bit in opcode BRAMs always in 32-bit port mode, fedcba9876543210 ================ .xxxxxxxxxx00000 - requires 10-bit address Address register, fedcba9876543210 access ================ ====== 00....xxxxxxxxxx 1024 x 32-bit 01...xxxxxxxxxxx 2048 x 16-bit 10..xxxxxxxxxxxx 4096 x 8-bit 11.xxxxxxxxxxxxx 8192 x 4-bit (supported for read only) Address register value to BRAM address translation Uses a 6:1 function for each bit, bits meaning ==== ======= 4 address shifted left {0,1,2,3} bits 2 the 'fe' address bits LUT area estimate, LUTs usage ==== ===== 8 optional XOR (16-bits at 2-bits per LUT), rounding up for ending register 10 translate (10-bits x 1 LUT) ---- ----- 18 total =========================== BRAM OUTPUT GENERATION =========================== Shifts DSP p output for store, and compute byte write mask Placement in pipeline, stage action ===== ====== 0 1 2 3 LUT new output here Permutations (showing address and byte write mask for store), 11111111111111110000000000000000 fedcba9876543210fedcba9876543210 ================================ aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa - 32-bit adr=00....xxxxxxxxxx write=1111 ................bbbbbbbbbbbbbbbb - 16-bit adr=01...xxxxxxxxxx0 write=0011 cccccccccccccccc................ - 16-bit adr=01...xxxxxxxxxx1 write=1100 ........................dddddddd - 8-bit adr=10..xxxxxxxxxx00 write=0001 ................eeeeeeee........ - 8-bit adr=10..xxxxxxxxxx01 write=0010 ........ffffffff................ - 8-bit adr=10..xxxxxxxxxx10 write=0100 gggggggg........................ - 8-bit adr=10..xxxxxxxxxx11 write=1000 ............................hhhh - 4-bit adr=11.xxxxxxxxxx000 ........................iiii.... - 4-bit adr=11.xxxxxxxxxx001 ....................jjjj........ - 4-bit adr=11.xxxxxxxxxx010 ................kkkk............ - 4-bit adr=11.xxxxxxxxxx011 ............llll................ - 4-bit adr=11.xxxxxxxxxx100 ........mmmm.................... - 4-bit adr=11.xxxxxxxxxx101 ....nnnn........................ - 4-bit adr=11.xxxxxxxxxx110 oooo............................ - 4-bit adr=11.xxxxxxxxxx111 Shift value for store, Requires 3:1 MUX per bit, 32 LUTs Generate write enable for store, Requires same 4-bits per function, 2 lower address bits 2 upper address bits 2 LUTs (5:1 function sharing inputs, 2 outputs per LUT) LUT area estimate, LUTs usage ==== ===== 32 shift value for store 2 generate write enable ---- ----- 34 total =================== PROGRAM COUNTER =================== 10-bit program counter (PC) Only lower 8-bits of PC increment on linear execution Requires only an 8-bit PC+1 computation (one slice) Placement in pipeline, stage action ===== ====== 0 register 1 register 2 increment PC 3 LUT new PC based on DSP p output and instruction opcode PC function inputs per output bit (map to 13:1 function at 2 LUTs/bit), bits meaning ==== ======= 1 next PC if not branching (computed in prior stage) 1 top of return stack 1 immediate absolute branch address 1 DSP P output register (computed branch target in prior clock) 1 DSP P output register sign bit (for conditional branch) 3 bits from instruction opcode LUT area estimate, LUTs usage ==== ===== 20 13:1 function for next 10-bit PC computation including instruction decode 8 PC+1 adder for 8 lower bits of PC 10 for 2 stage registers (2-bits/LUT) ---- ----- 38 total ======================== INSTRUCTION PIPELINE ======================== Todo, remember to count cost to pipeline opcode bits through stages ========= NOTES ========= ================================================= ADDRESS REGISTER XOR INSTEAD OF ADD IMMEDIATE ================================================= Planning on [address ^ immediate] addressing This removes an adder from the design XOR is the same as [address + immediate] for an n-bit immediate When lower n-bits of address are zero Means data must be aligned to the nearest pow2 of maximum immediate offset Using the following terms, ggggggoooo g = group bits (address bits choose the group of data) o = offset bits (address bits are zero, immediate chooses element in group) For bits in address which are not cleared (ie the group bits), Setting bits in the immediate results in accessing a neighbor group Regardless of the starting group in the address register It is possible to roll through all aligned groups But ordering is different based on starting group address Example of group bits for address crossed with immediate 00 01 10 11 +------------- 00 | 00 01 10 11 01 | 01 00 11 10 10 | 10 11 00 01 11 | 11 10 01 00 =================================== BRAM VARIABLE BIT-WIDTH WINDOWS =================================== Trying to support transparent pack/unpack of variable bit-widths from BRAM Want zero impact to ISA, no special instructions Instead dividing address range into windows of different bit-widths Each address range addresses at a multiple of the bit-width Effectively the high bits of address choose the bit-width Store path limited to {8,16,32}-bit Only using BRAM byte write mask to avoid any {read, modify, write} Fixed signed vs unsigned configuration, size choice ====== ====== 32-bit signed (but doesn't matter) 16-bit going to go with signed (needed for vector or audio) 8-bit unsigned (keeps implementation simple) 4-bit unsigned for sure (sprites?) ========================================== WORKING THROUGH OPTIONS DSP OPERATIONS ========================================== Opcode forms, p = c op ((a << 16) + unsigned(b)) p = c + (a * b) p = c - (a * b) Where op can be the following, and ..... nand .... nor ..... not ..... or ...... xnor .... xor ..... Where the following can also be applied, extra set c bit -1 to 1 (for rounding) (a * b) can be forced to zero (nop) ((a << 16) + unsigned(b)) can be forced to zero (nop) ((a << 16) + unsigned(b)) can be forced to all ones =============================================== WORKING THROUGH OPTIONS FOR A,B,C DSP INPUT =============================================== DSP inputs (as they appear in the core), 24-bit a 16-bit b 40-bit c Possible inputs, 10-bit immediate 16-bit top of return stack 32-bit top of data stack 32-bit register file load (from prior instruction) 32-bit BRAM load (from prior instruction) 40-bit DSP p output ==================== FAST ABS MIN MAX ==================== Simple design exercise to think through DSP issues These need to work on the 40-bit accumulator without precision loss So using multiply stage is out Min and max, where a is the accumulator, and b is the limit, min(a, b) = ((a - b) & ((a - b) < 0 ? ~0 : 0)) + b max(a, b) = ((a - b) & ((a - b) < 0 ? 0 : ~0)) + b Want to be able do the following, acc -= b; acc = acc < 0 ? acc : 0; // want to fold this into prior operation acc += b; Have to either LUT or register p in stage 3, Could LUT p to zero if signed or unsigned based on control bit This works out to 2-bits/LUT (pair of 5:1 functions with same input) So 20 LUTs total (same as just registering) Plus likely need to decode control and enable from opcode in prior pass inputs ------ 2 p bits 1 p sign bit (might want the overflow sign bit?) 1 enable bit 1 signed or unsigned control bit This enables min and max to work in 2 instructions without branching Absolute value, abs(a) = max(a, -a) Does this make the case for, expanding data stack to 40-bit (to match accumulator)? reducing accumulator to 36-bit, or even 32-bit? Operation, push copy of acc; // want to fold into start of next op acc += acc; acc = acc < 0 ? 0 : acc; acc -= pop; Using top of data stack for DSP input means it must be pre-registered That register could be 40-bit until it gets actually stored on stack Want a bit which marks if should consume data stack Ideally push to happen before the first add (included in that opcode) Stage 0 : must save top to stack RAM Stage 1 : set top to p Todo, think through when c is computed again Data stack top is going to be expensive 40-bit : minimum 80 LUTs 32-bit : minimum 64 LUTs Time to rethink ...

**Related**

Avnet AES-KU040-DB-G (XCKU040 Based Dev Board)

## No comments:

## Post a Comment