Displacement Register | Texas Instruments TMS320C67X/C67X+ DSP

Mapping Between Instruction and Functional Unit

Table B−1. Functional Unit to Instruction Mapping (Continued)

		Functional Unit

Instruction	.L Unit	.M Unit	.S Unit	.D Unit
B displacement			n
B register			n†
B IRP			n†
B NRP			n†
CLR			n
CMPEQ	n
CMPEQDP			n
CMPEQSP			n
CMPGT	n
CMPGTDP			n
CMPGTSP			n
CMPGTU	n
CMPLT	n
CMPLTDP			n
CMPLTSP			n
CMPLTU	n
DPINT	n
DPSP	n
DPTRUNC	n
EXT			n
EXTU			n
IDLE

†S2 only

‡ D2 only

§ C67x+ DSP-specific instruction

B-2

Mapping Between Instruction and Functional Unit

SPRU733

Image 429

Texas Instruments TMS320C67X/C67X+ DSP manual Displacement Register

Contents

TMS320C67x/C67x+ DSP CPU and Instruction Set Reference Guide Copyright 2005, Texas Instruments Incorporated Read This First Trademarks Contents Instruction Set Contents Vii Mvkl Move Signed Constant Into Register Pipeline Interrupts SPRU733 Figures −18 Tables 131 Tables Examples Introduction TMS320C6000 DSP Family Overview TMS320 DSP Family Overview Automotive Consumer Control −1. Typical Applications for the TMS320 DSPsGeneral-Purpose Graphics/Imaging Industrial Instrumentation Medical Military TMS320C67x DSP Features and Options SPRU733 TMS320C67x DSP Features and Options TMS320C67x DSP Architecture −1. TMS320C67x DSP Block Diagram Central Processing Unit CPU Internal MemoryMemory and Peripheral Options SPRU733 CPU Data Paths and Control General-Purpose Register Files Introduction −1. TMS320C67x CPU Data Paths Register Files Devices −1 -Bit/64-Bit Register Pairs −2. Functional Units and Operations Performed Functional Units Memory, Load, and Store Paths Register File Cross Paths Control Register File Data Address Paths−3. Control Registers Acronym Register Name Section Register Addresses for Accessing the Control Registers −4. Register Addresses for Accessing the Control RegistersAcronym Register Name Address Read/ Write Pipeline Stage Pipeline/Timing of Control Register Accesses Addressing Mode Register AMR −5. Addressing Mode Register AMR Field DescriptionsBit Field Value Description B6 Mode BK n Value Block Size −6. Block Size Calculations −4. Control Status Register CSR Control Status Register CSR Bit Field −7. Control Status Register CSR Field Descriptions DCC −8. Interrupt Clear Register ICR Field Descriptions Interrupt Clear Register ICR −9. Interrupt Enable Register IER Field Descriptions Interrupt Enable Register IER −10. Interrupt Flag Register IFR Field Descriptions Interrupt Flag Register IFR −9. Interrupt Return Pointer Register IRP Interrupt Return Pointer Register IRP −11. Interrupt Set Register ISR Field Descriptions Interrupt Set Register ISR −11.Interrupt Service Table Pointer Register Istp Interrupt Service Table Pointer Register Istp Nonmaskable Interrupt NMI Return Pointer Register NRP 12 E1 Phase Program Counter PCE1 Floating-Point Adder Configuration Register Fadcr Control Register File Extensions−13. Control Register File Extensions −14. Floating-Point Adder Configuration Register Fadcr NAN2 Inexact results status for .L1 −15. Floating-Point Auxiliary Configuration Register Faucr Floating-Point Auxiliary Configuration Register Faucr UND NaN select for .S2 src2 Signed infinity for .S1 −16. Floating-Point Multiplier Configuration Register Fmcr Floating-Point Multiplier Configuration Register Fmcr Inexact results status for .M2 Rounding mode select for .M1 Denormalized number select for .M1 src1 Topic Instruction Set Symbol Meaning Instruction Operation and Execution Notations−1. Instruction Operation and Execution Notations Rotl Occurs Extu l,r Greater than −2. Instruction Syntax and Opcode Notations Instruction Syntax and Opcode Notations Ucstn Bit unsigned constant field Ucst n SPRU733 Sdfpn −3. Ieee Floating-Point Notations −4. Special Single-Precision Values Symbol Sign s Exponent e Fraction f −2. Double-Precision Floating-Point Fields Symbol Hex Value Decimal Value −6. Special Double-Precision Values Delay Slots Instruction Type Slots Unit Latency Read Cycles† −8. Delay Slot and Functional Unit Latency −3. Basic Format of a Fetch Packet Parallel Operations Example 3−2. Fully Parallel p-Bit Pattern in a Fetch Packet Example 3−1. Fully Serial p-Bit Pattern in a Fetch PacketCycle/Execute Instructions Example Parallel Code Branching Into the Middle of an Execute PacketCycle/Execute Packet Instructions −9. Registers That Can Be Tested by Conditional Operations Conditional OperationsSpecified Conditional Bit Register Resource Constraints Constraints on Instructions Using the Same Functional Unit Constraints on Cross Paths 1X Following execute packets are valid Constraints on Loads and Stores Following code sequence is invalid Constraints on Long 40-Bit Data Constraints on Register Reads However, this code sequence is valid Constraints on Register Writes MPYSP2DP Constraints on Floating-Point Instructions Intdp Tional unit on cycle i + 4, i + 5, or i + Addsp Subsp Spint Sptrunc Intsp Mpysp Addressing Modes Linear Addressing Mode Example 3−4. LDW Instruction in Circular Mode Circular Addressing ModeBefore LDW Cycle after LDW Cycles after LDW Syntax for Load/Store Address Generation Example 3−5. Addah Instruction in Circular ModeBefore Addah Cycle after Addah −10. Indirect Address Generation for Load/Store Mode Field Syntax Modification Performed−11. Address Generator Options for Load/Store Addressing Type Address Register Instruction Descriptions Instruction Compatibility Example Way each instruction is described Opcode map field used For operand type Unit Opfield Execution for .L1, .L2 and .S1, .S2 Opcodes Absolute Value With Saturation ABSPipeline StageE1 Read src2 Written dst Unit in use Before instruction ABSDP, AbsspCycle after instruction Before instruction Cycle after instruction Absdp Absolute Value, Double-Precision Floating-Point Pipeline Stage Read ABS, AbsspWritten Before instruction Cycles after instruction Abssp Absolute Value, Single-Precision Floating-Point ABS, Absdp ADD Add Two Signed Integers Without Saturation ADD Unit in use Or .D ADDDP, ADDK, ADDSP, ADDU, ADD2, SADD, SUB Add Two Signed Integers Without Saturation ADD Add Using Byte Addressing Mode AddabADD, ADDAD, ADDAH, Addaw Add Using Byte Addressing Mode Addab Addad Add Using Doubleword Addressing Mode ADD, ADDAB, ADDAH, Addaw Add Using Halfword Addressing Mode AddahADD, ADDAB, ADDAD, Addaw Add Using Halfword Addressing Mode Addah Add Using Word Addressing Mode AddawADD, ADDAB, ADDAD, Addah Add Using Word Addressing Mode Addaw Adddp Add Two Double-Precision Floating-Point Values Add Two Double-Precision Floating-Point Values Adddp Unit in use Or .S ADD, ADDSP, ADDU, Subdp Add Signed 16-Bit Constant to Register AddkPipeline StageE1 Read cst16 Written dst Unit in use Addsp Add Two Single-Precision Floating-Point Values Add Two Single-Precision Floating-Point Values Addsp ADD, ADDDP, ADDU, Subsp Add Two Unsigned Integers Without Saturation AdduADD, SADD, Subu Addu Add Two Unsigned Integers Without Saturation ADD2 Add Two 16-Bit Integers on Upper and Lower Register Halves ADD, ADDU, SUB2 Bitwise OR, XOR Branch Using a Displacement Opcode map field used For operand type UnitS1, .S2 Cycle Program Counter Value Action Delay Slots Example Xuint Branch Using a Register Written Branch Taken Unit in use Target Instruction Pipeline Stage Read Branch Using an Interrupt Return Pointer IRPXsint −15. Program Counter Values for B IRP Instruction NRP Branch Using NMI Return Pointer −16. Program Counter Values for B NRP Instruction CLR Clear a Bit Field SET Execution Pipeline Clear a Bit Field CLR Cmpeq Compare for Equality, Signed Integers CMPEQDP, CMPEQSP, CMPGT, Cmplt Cmpeqdp Compare for Equality, Double-Precision Floating-Point Values Cmpeqdp .S1 Cmpeqsp Compare for Equality, Single-Precision Floating-Point Values CMPEQ, CMPEQDP, CMPGTSP, Cmpltsp Cmpgt Compare for Greater Than, Signed Integers CMPEQ, CMPGTDP, CMPGTSP, CMPGTU, Cmplt Cmpgt .L1X Cmpgtdp Delay Slots Functional Unit Latency See Also Example Cmpgtsp CMPEQSP, CMPGT, CMPGTDP, CMPGTU, Cmpltsp Cmpgtu Compare for Greater Than, Unsigned Integers CMPGT, CMPGTDP, CMPGTSP, Cmpltu Cmplt Compare for Less Than, Signed Integers Cmplt Compare for Less Than, Signed Integers Compare for Less Than, Signed Integers Cmplt Wise, 0 is written to dst CMPEQDP, CMPGTDP, CMPLT, CMPLTSP, Cmpltu Cmpltsp Cmpltsp .S1 A1,A2,A3 Cmpltu Compare for Less Than, Unsigned Integers Instruction Type Single-cycle Delay Slots See Also Example Dpint Convert Double-Precision Floating-Point Value to Integer Example Dpint Floating-Point Value Dpsp DPINT, DPTRUNC, INTSP, Spdp With Truncation Dptrunc DPINT, DPSP, Sptrunc Dptrunc EXT Extract and Sign-Extend a Bit Field If cond src2 ext csta, cstb → dst else nop Extu Extu Extract and Zero-Extend a Bit Field If cond src2 extu csta, cstb → dst else nop EXT Multicycle NOP With No Termination Until Interrupt IdleIdle DPINT, INTDPU, INTSP, Intspu Intdp Intdp INTDP, INTSP, Intspu Intdpu Intdpu INTDP, INTDPU, Intspu Intsp INTDP, INTDPU, Intsp Intspu Register Offset Ldbu−17. Data Types Supported by Ldbu Instruction Left Shift LDH, LDW Before LDB Cycle after LDBCycles after LDB −18. Data Types Supported by Ldbu Instruction 15-Bit Offset Load Byte From Memory With a 15-Bit Unsigned Constant Offset Before LDB Cycle after LDB Pipeline Stage Read B14 / B15 Written Or Register Offset Lddw Execution Pipeline Instruction Type LDB, LDH, LDW −19. Data Types Supported by Ldhu Instruction Ldhu LDB, LDW Before LDH Cycle after LDHCycles after LDH Tion operates only on the .D2 unit −20. Data Types Supported by Ldhu Instruction 15-Bit Offset LDW LDB, LDDW, LDH Cycle after LDW LDW LDB, LDH Leftmost Bit Detection Lmbd → dst MPY Multiply Signed 16 LSB y Signed 16 LSBMPYU, MPYSU, MPYUS, Smpy Pipeline StageE1 E2 Read MPY Multiply Signed 16 LSB x Signed 16 LSB Mpydp Multiply Two Double-Precision Floating-Point Values Pipeline E10 Stage Read MPY, Mpysp MPYHU, MPYHSU, MPYHUS, Smpyh Multiply Signed 16 MSB y Signed 16 MSB Mpyh Multiply Signed 16 MSB x Signed 16 MSB Multiply Signed 16 MSB y Signed 16 LSB MpyhlMPYHLU, MPYHSLU, MPYHULS, Smpyhl Mpyhl Multiply Signed 16 MSB x Signed 16 LSB MPYHL, MPYHSLU, Mpyhuls Multiply Unsigned 16 MSB y Unsigned 16 LSB Mpyhslu Multiply Signed 16 MSB y Unsigned 16 LSBMPYHL, MPYHLU, Mpyhuls MPYH, MPYHU, Mpyhus Multiply Signed 16 MSB y Unsigned 16 MSB Mpyhu Multiply Unsigned 16 MSB y Unsigned 16 MSBMPYH, MPYHSU, Mpyhus Multiply Unsigned 16 MSB y Signed 16 LSB MpyhulsMPYHL, MPYHLU, Mpyhslu MPYH, MPYHU, Mpyhsu Multiply Unsigned 16 MSB y Signed 16 MSB Multiply 32-Bit y 32-Bit Into 32-Bit Result MpyiMpyi Mpyid Mpyid Multiply 32-Bit y 32-Bit Into 64-Bit Result Mpyid Multiply 32-Bit x 32-Bit Into 64-Bit Result Multiply Signed 16 LSB y Signed 16 MSB MpylhMPYLHU, MPYLSHU, MPYLUHS, Smpylh Mpylh Multiply Signed 16 LSB x Signed 16 MSB MPYLH, MPYLSHU, Mpyluhs Multiply Unsigned 16 LSB y Unsigned 16 MSB Mpylshu Multiply Signed 16 LSB y Unsigned 16 MSBMPYLH, MPYLHU, Mpyluhs Multiply Unsigned 16 LSB y Signed 16 MSB MpyluhsMPYLH, MPYLHU, Mpylshu Mpysp Multiply Two Single-Precision Floating-Point Values MPY, MPYDP, MPYSP2DP Mpysp Mpyspdp MPY, MPYDP, MPYSP, MPYSP2DP Mpyspdp Multiply Two Single-Precision Floating-Point Values for MPYSP2DPDouble-Precision Result MPYSP2DP Multiply Signed 16 LSB y Unsigned 16 LSB Mpysu MPY, MPYU, Mpyus MPY, MPYSU, Mpyus Multiply Unsigned 16 LSB y Unsigned 16 LSB Multiply Unsigned 16 LSB x Unsigned 16 LSB Mpyu Mpyus Multiply Unsigned 16 LSB y Signed 16 LSBMPY, MPYU, Mpysu Multiply Unsigned 16 LSB x Signed 16 LSB Mpyus Move From Register to Register If cond 0 + src2 → dst MVC Move Between Control File and Register File Src2 → dst −21. Register Addresses for Accessing the Control Registers Move Signed Constant Into Register and Sign Extend MVKPipeline StageE1 Read Written dst Unit in use MVKH, MVKL, Mvklh MVKH/MVKLH Move 16-Bit Constant Into Upper Bits of Register If you are loading the address of a label, use Mvkl Pipeline Stage Read Written MVK, MVKH, Mvklh NEG Negate No Operation NOPUcst4 None Cycle after NOP Before NOPNo operation Executes Cycle after ADD Norm Normalize Integer Execution If cond Norm src → dst Else nop Pipeline Not Bitwise not Bitwise or AND, XOR Src1 or src2 → dst Rcpdp Double-Precision Floating-Point Reciprocal Approximation RCPSP, Rsqrdp Rcpsp Single-Precision Floating-Point Reciprocal Approximation RCPDP, Rsqrsp Rsqrdp If src2 is positive infinity, positive 0 is placed in dst Rsqrsp Rsqrsp .S1 Sadd Add Two Signed Integers With Saturation ADD, Ssub Add Two Signed Integers With Saturation Sadd SAT Saturate a 40-Bit Integer to a 32-Bit Integer SAT .L2 SET Set a Bit Field CLR If cond src2 SET csta, cstb → dst else nop SET .S1 SHL Arithmetic Shift Left SHR, Sshl SHR Arithmetic Shift Right SHL, Shru Shru Logical Shift Right SHL, SHR MPY, SMPYH, SMPYHL, Smpylh Smpy CSR MPYH, SMPY, SMPYHL, Smpylh Smpyh MPYHL, SMPY, SMPYH, Smpylh Smpyhl Instruction Set 223 MPYLH, SMPY, SMPYH, Smpyhl Smpylh Instruction Set 225 Spdp DPSP, INTDP, SPINT, Sptrunc Spint Convert Single-Precision Floating-Point Value to Integer DPINT, INTSP, SPDP, Sptrunc Spint Sptrunc DPTRUNC, SPDP, Spint Sptrunc Sshl Shift Left With Saturation Sshl .S1 Ssub Subtract Two Signed Integers With Saturation SUB STB Before Cycle after Instruction STH, STW Store Byte to Memory With a 15-Bit Unsigned Constant Offset Pipeline Stage Read B14 /B15 , src Written Unit in use STH Before STB, STW Instruction Cycles after STH Instruction Type Store Delay Slots See Also STW STB, STH Store Word to Memory With a 15-Bit Unsigned Constant Offset 248 SUB Subtract Two Signed Integers Without Saturation Src1 − src2 → dst else nop Src2 − src1 → dst ADD, SSUB, SUBC, SUBDP, SUBSP, SUBU, SUB2 Subtract Using Byte Addressing Mode SubabSUB, SUBAH, Subaw BK0 = 3 → size = A5 in circular addressing mode using BK0 Subtract Using Halfword Addressing Mode SubahSUB, SUBAB, Subaw Subtract Using Word Addressing Mode SubawSUB, SUBAB, Subah Subtract Using Word Addressing Mode Subaw Subtract Conditionally and Shift-Used for Division SubcADD, SSUB, SUB, SUBDP, SUBSP, SUBU, SUB2 Subtract Conditionally and Shift−Used for Division Subc Subdp Subtract Two Double-Precision Floating-Point Values Subtract Two Double-Precision Floating-Point Values Subdp ADDDP, SUB, SUBSP, Subu Subsp Subtract Two Single-Precision Floating-Point Values Subsp Subtract Two Single-Precision Floating-Point Values ADDSP, SUB, SUBDP, Subu Subtract Two Unsigned Integers Without Saturation SubuADDU, SSUB, SUB, SUBC, SUBDP, SUBSP, SUB2 Subtract Two Unsigned Integers Without Saturation Subu Src1 and placed in the lower-half of dst SUB2 ADD2, SSUB, SUB, SUBC, Subu XOR Bitwise Exclusive or AND, or Zero Zero a Register Pipeline Fetch Pipeline Operation Overview Decode −2. Fetch Phases of the Pipeline −3. Decode Phases of the Pipeline Execute −4. Execute Phases of the Pipeline Clock cycle Fetch Pipeline Operation Summary −1. Operations Occurring During Pipeline Phases Stage Phase Symbol During This Phase Completed Adddp −7. Pipeline Phases Block Diagram Example 4−1. Execute Packet in −7 Pipeline Execution of Instruction Types Delay slots Functional Unit latency DP Compare Instruction Type Execution Phases Cycle DP ADDDP/SUBDP Mpyi Mpyid Mpydp Instruction Type Execution Phases Instruction Type Execution Phases Single-Cycle Instructions −3. Single-Cycle Instruction ExecutionUnit in use M, or .D −4 y 16-Bit Multiply Instruction Execution 2 16 y 16-Bit Multiply Instructions −5. Store Instruction Execution Store Instructions −13. Store Instruction Execution Block Diagram −6. Load Instruction Execution Load Instructions −15. Load Instruction Execution Block Diagram −7. Branch Instruction Execution Branch Instructions −17. Branch Instruction Execution Block Diagram −8. Two-Cycle DP Instruction Execution Two-Cycle DP Instructions Four-Cycle Instructions −9. Four-Cycle Instruction ExecutionUnit in use Or .M −10. Intdp Instruction Execution Intdp Instruction −11. DP Compare Instruction Execution DP Compare Instructions −12. ADDDP/SUBDP Instruction Execution ADDDP/SUBDP Instructions −13. Mpyi Instruction Execution Mpyi Instruction Mpyid Instruction −14. Mpyid Instruction ExecutionPipeline Stage E4 E5 E6 E7 E8 E9 E10 Read −15. Mpydp Instruction Execution Mpydp Instruction −16. Mpyspdp Instruction Execution Mpyspdp Instruction Functional Unit Constraints MPYSP2DP Instruction−17. MPYSP2DP Instruction Execution Unit Constraints −18. Single-Cycle .S-Unit Instruction ConstraintsInstruction Execution −19. DP Compare .S-Unit Instruction Constraints −20 -Cycle DP .S-Unit Instruction Constraints −21. ADDSP/SUBSP .S-Unit Instruction Constraints −22. ADDDP/SUBDP .S-Unit Instruction Constraints −23. Branch .S-Unit Instruction Constraints −24 y 16 Multiply .M-Unit Instruction Constraints −25 -Cycle .M-Unit Instruction Constraints −26. Mpyi .M-Unit Instruction Constraints −27. Mpyid .M-Unit Instruction Constraints −28. Mpydp .M-Unit Instruction Constraints −29. Mpysp .M-Unit Instruction Constraints −30. Mpyspdp .M-Unit Instruction Constraints −31. MPYSP2DP .M-Unit Instruction Constraints −32. Single-Cycle .L-Unit Instruction Constraints −33 -Cycle .L-Unit Instruction Constraints −34. Intdp .L-Unit Instruction Constraints −35. ADDDP/SUBDP .L-Unit Instruction Constraints −36. Load .D-Unit Instruction Constraints Unit Instruction Constraints −37. Store .D-Unit Instruction Constraints −38. Single-Cycle .D-Unit Instruction Constraints Lddw Performance Considerations Pipeline Stall −29. Multicycle NOP in an Execute Packet Multicycle NOPs Cycle # Pipeline Phase Branch Target Memory Considerations −40. Program Memory Accesses Versus Data Load AccessesData Load Operation +10 −32. Program and Data Memory Stalls Example 4−2. Load From Memory Banks −33 -Bank Interleaved Memory −34 -Bank Interleaved Memory With Two Memory Spaces −41. Loads in Pipeline from Example 4−2 Interrupts Overview Types of Interrupts and Signals Used Priority Interrupt Name Interrupt Type −1. Interrupt Priorities Nonmaskable Interrupt NMI Interrupt Acknowledgment Iack and Interrupt Number INUMn −1. Interrupt Service Table Interrupt Service Table IST −2. Interrupt Service Fetch Packet 1238h Interrupt Service Table Pointer Istp Example 5−1. Relocation of Interrupt Service Table Summary of Interrupt Control Registers −2. Interrupt Control RegistersAcronym Register Name Description Globally Enabling and Disabling Interrupts MVC CSR,B0 Enabling and Disabling Interrupts Individual Interrupt Control Status of Interrupts Setting and Clearing Interrupts Returning From Interrupt Servicing Example 5−8. Code to Return From NMIExample 5−9. Code to Return from a Maskable Interrupt Setting the Nonreset Interrupt Flag Conditions for Processing a Nonreset InterruptInterrupt Detection and Processing Isfp Actions Taken During Nonreset Interrupt Processing PS PW PR DP DC Setting the Reset Interrupt Flag Actions Taken During Reset Interrupt Processing Pipeline Interaction General Performance Single Assignment Programming Programming Considerations Nested Interrupts Example 5−11. Code Using Single Assignment STW Example 5−14. Manual Interrupt Processing Manual Interrupt Processing Traps Example 5−15. Code Sequence to Invoke a TrapExample 5−16. Code Sequence for Trap Return Instruction C62x DSP C64x DSP C67x DSP C67x+ DSP Instruction Compatibility Cmpltu Intsp Mpylh Rsqrdp Subab Functional Unit Instruction Table B−1. Functional Unit to Instruction Mapping Displacement Register Intdp Intdpu Intsp Intspu N n n n n n n n n n n n n STB memory SUB Subab Subah Subaw Subc Subdp Subsp Subu SUB2 XOR Zero Unit Instructions and Opcode Maps Table C−1. Instructions Executing in the .D Functional Unit Instructions Executing in the .D Functional Unit Table C−2. .D Unit Opcode Map Symbol Definitions Opcode Map Symbols and Meanings Syntax Modification Performed Table C−3. Address Generator Options for Load/Store Figure C−1 or 2 Sources Instruction Format 32-Bit Opcode Maps Appendix D Table D−1. Instructions Executing in the .L Functional Unit Instructions Executing in the .L Functional Unit Table D−2. .L Unit Opcode Map Symbol Definitions Figure D−1 or 2 Sources Instruction Format Appendix E Table E−1. Instructions Executing in the .M Functional Unit Instructions Executing in the .M Functional Unit Table E−2. .M Unit Opcode Map Symbol Definitions Figure E−1. Extended M-Unit with Compound Operations Appendix F Table F−1. Instructions Executing in the .S Functional Unit Instructions Executing in the .S Functional Unit Table F−2. .S Unit Opcode Map Symbol Definitions Figure F−1 or 2 Sources Instruction Format Figure F−7. Branch with NOP Constant Instruction Format No Unit Specified Instructions and Opcode Maps Table G−1. Instructions Executing With No Unit Specified Instructions Executing With No Unit Specified Figure G−1. Loop Buffer Instruction Format Index Cmpeqdp Dpsp Index-4 Reset Index-6 Mpysu Unit No unit instructions Rsqrdp To memory with a 15-bit unsigned constant offset STB 260 Single-precision Subsp 263