Western Digital WD16
WD16 microprocessor in five 40-pin packages installed on an Alpha Microsystems AM-100 CPU board which connects to a S-100 bus.
General information
Launched1976 (1976)
Common manufacturer(s)
Performance
Max. CPU clock rateto 3.3 MHz
Data width16
Address width16
Architecture and classification
Instruction setPDP-11 like
Number of instructions119
Physical specifications
Package(s)
History
Successor(s)none

The WD16 is a 16-bit microprocessor introduced by Western Digital in October 1976. It is based on the MCP-1600 chipset, a general-purpose design that was also used to implement the DEC LSI-11 low-end minicomputer and the Pascal MicroEngine processor. The three systems differed primarily in their microcode, giving each system a unique instruction set architecture (ISA).

The WD16 implements an extension of the PDP-11 instruction set architecture but is not machine code compatible with the PDP-11.[1] The instruction set and microcoding were created by Dick Wilcox and Rich Notari.[2] The WD16 is an example of orthogonal CISC architecture. Most two-operand instructions can operate memory-to-memory with any addressing mode and some instructions can result in up to ten memory accesses.

The WD16 is implemented in five 40-pin DIP packages. Maximum clock speed is 3.3 MHz. Its interface to memory is via a 16-bit multiplexed data/address bus.[3]

The WD16 is best known for its use in Alpha Microsystems' AM-100 and AM-100/T processor boards.[4] A prototype was demonstrated in 1977.[5] As of 1981 there were at least 5,000 Alpha Micro computers based on the WD16.[6] As late as 1982, WD16-based Alpha Micros were still being characterized as "supermicros."[7] The WD16 was superseded by the Motorola 68000 in June 1982.[8]

Memory

Data formats

The smallest unit of addressable and writable memory is the 8-bit byte. Bytes can also be held in the lower half of registers R0 through R5.[9]

16-bit words are stored little-endian with least significant bytes at the lower address. Words are always aligned to even memory addresses. Words can be held in registers R0 through R7.

32-bit double words can only be stored in register pairs with the lower word being stored in the lower-numbered register. 32 bit values are used by MUL, DIV and some rotate and arithmetic shift instructions.

Floating point values are 48 bits long and can only be stored in memory. This format is half-way between single and double precision floating point formats. They are stored an unusual middle-endian format sometimes referred to as "PDP-endian." Floating point values are always aligned to even addresses. The first word contains the sign, exponent, and high byte of the mantissa. The next higher address contains the middle two bytes of the mantissa, and the next higher address contains the lowest two bytes of the mantissa. The complete format is as follows:

1. A 1 bit sign for the entire number which is zero for positive.

2. An 8-bit base-two exponent in excess-128 notation with a range of +127, -128. The only legal number with an exponent of -128 is true zero (all zeros).

3. A 40 bit mantissa with the MSB implied.

15 14 7 6 0
Addr+0 S Exponent Mantissa (high)
15 8 7 0
Addr+2 Mantissa (middle)
15 8 7 0
Addr+4 Mantissa (low)

Memory management

The WD16's 16-bit addresses can directly access 64 KB of memory. The WD16 does not offer any inherent memory management or protection. In the AM-100 application, the last 256 memory locations are mapped to port space.[10] As most AM-100 computers were used as multi-user computers, the memory would usually be expanded past 64K with bank switching. Although the AM-100 could be configured for up to 22 users and 512 Kilobytes of RAM,[6] a typical memory configuration for a 9-user AM-100 might be in the range of 352 Kilobytes.[11] In 1981 an optional AM-700 memory management unit was offered for the AM-100/T which allowed memory segmentation in 256 byte increments.[12]

CPU registers

WD16 registers
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00 (bit position)
Main registers
R0 Register 0 / Count
R1 Register 1
R2 Register 2
R3 Register 3
R4 Register 4
R5 Register 5
Stack pointer
R6 / SP Register 6 / Stack Pointer
Program counter
R7 / PC Register 7 / Program Counter
Status flags
  ALU N Z V C Processor Status Word

The CPU contains eight general-purpose 16-bit registers, R0 to R7. The registers can be used for any purpose with these exceptions: Register R7 is the program counter (PC). Although any register can be used as a stack pointer, R6 is the stack pointer (SP) used for hardware interrupts and traps. R0 is the count for the block transfer instructions.[9]

Addressing modes

Most instructions allocate six bits to specify each operand. Three bits select one of eight addressing modes and three bits select a general register. The encoding of the six bit operand addressing mode is as follows:[9]

5 3 2 0
Mode Register


In the following sections, each item includes an example of how the operand would be written in assembly language. Rn means one of the eight registers, written R0 through R7.

General register addressing modes

The following eight modes can be applied to any general register. Their effects when applied to R6 (the stack pointer, SP) and R7 (the program counter, PC) are set out separately in the following sections.

Code Name Example Description
0n Register Rn The operand is in Rn
1n Register deferred (Rn) Rn contains the address of the operand
2n Autoincrement (Rn)+ Rn contains the address of the operand, then increment Rn
3n Autoincrement deferred @(Rn)+ Rn contains the address of the address of the operand, then increment Rn by 2
4n Autodecrement −(Rn) Decrement Rn, then use the result as the address of the operand
5n Autodecrement deferred @−(Rn) Decrement Rn by 2, then use the result as the address of the address of the operand
6n Index X(Rn) Rn+X is the address of the operand
7n Index deferred @X(Rn) Rn+X is the address of the address of the operand

In index and index deferred modes, X is a 16-bit value taken from a second word of the instruction. In double-operand instructions, both operands can use these modes. Such instructions are three words long.

Autoincrement and autodecrement operations on a register are by 1 in byte instructions, by 2 in word instructions, and by 2 whenever a deferred mode is used, since the quantity the register addresses is a (word) pointer.

Program counter addressing modes

When R7 (the program counter) is specified, four of the addressing modes naturally yield useful effects:

Code Name Example Description
27 Immediate #n The operand is the next word of the instruction
37 Absolute @#a The address of the operand is the next word of the instruction
67 Relative a The address of the operand is the next word of the instruction added to the PC
77 Relative deferred @a The address of the address of the operand is the next word of the instruction added to PC

There are only two common uses of absolute mode, whose syntax combines immediate and deferred mode. The first is accessing the reserved processor locations at 0000-003F. The other is to specify input/output registers in port space, as the registers for each device have specific memory addresses. Relative mode has a simpler syntax and is more typical for referring to program variables and jump destinations. A program that uses relative mode (and relative deferred mode) exclusively for internal references is position-independent; it contains no assumptions about its own location, so it can be loaded into an arbitrary memory location, or even moved, with no need for its addresses to be adjusted to reflect its location. In computing such addresses relative to the current location, the processor performs relocation on the fly.

Immediate and absolute modes are merely autoincrement and autoincrement deferred mode, respectively, applied to PC. When the auxiliary word is in the instruction, the PC for the next instruction is automatically incremented past the auxiliary word. As PC always points to words, the autoincrement operation is always by a stride of 2.

Stack addressing modes

R6, also written SP, is used as a hardware stack for traps and interrupts. A convention enforced by the set of addressing modes the WD16 provides is that a stack grows downward—toward lower addresses—as items are pushed onto it. When a mode is applied to SP, or to any register the programmer elects to use as a software stack, the addressing modes have the following effects:

Code Name Example Description
16 Deferred (SP) The operand is on the top of the stack
26 Autoincrement (SP)+ The operand is on the top of the stack, then pop it off
36 Autoincrement deferred @(SP)+ A pointer to the operand is on top of stack; pop the pointer off
46 Autodecrement −(SP) Push a value onto the stack
66 Indexed X(SP) The operand is located X distance from the top of stack
76 Indexed deferred @X(SP) The pointer to the operand is located X distance from the top of stack

Although software stacks can contain bytes, SP always points to a stack of words. Autoincrement and autodecrement operations on SP are always by a stride of 2.

Instruction set

Most of the WD16 instructions operate on bytes and words. Bytes are specified by a register number—identifying the register's low-order byte—or by a memory location. Words are specified by a register number or by the memory location of the low-order byte, which must be an even number. In the lists in the following sections, the letter B is appended to the instruction symbol to specify a byte operation, for example, MOV becomes MOVB. All opcodes and addresses are expressed in hexadecimal.[9]

Double-operand instructions

The high-order four bits specify the operation to be performed. Two groups of six bits specify the source operand addressing mode and the destination operand addressing mode, as defined above.

15 12 11 9 8 6 5 3 2 0
Opcode Src Register Dest Register
OpcodeMnemonicOperation
1000 ADDAdd: Dest ← Dest + Src
2000 SUBSubtract: Dest ← Dest - Src
3000 ANDAnd: Dest ← Dest ∧ Src
4000 BICBit clear: Dest ← Dest ∧ (-1 - Src)
5000 BISBit Set: Dest ← Dest ∨ Src
6000 XORExclusive or: Dest ← Dest ⊻ Src
9000 CMPCompare: Set-flags(Src − Dest)
A000 BITBit test: Set-flags(Dest ∧ Src)
B000 MOVMove: Dest ← Src
C000 CMPBCompare byte: Set-flags(Src − Dest)
D000 MOVBMove byte: Dest ← Src (Register destination sign-extends into bits 8-15)
E000 BISBBit set byte: Dest ← Dest ∨ Src

Some two-operand instructions utilize an addressing mode for one operand and a register for the second operand:

15 9 8 6 5 3 2 0
Opcode Reg Src/Dest Register

The high-order seven bits specify the operation to be performed, six bits specify the operand addressing mode and three bits specify a register or register pair. Where a register pair is used (written below as "Reg+1:Reg") Reg contains the low-order portion of the operand. The next higher numbered register contains the high-order portion of the operand (or the remainder).

OpcodeMnemonicOperation
7200 LEALoad effective address: Reg ← ea(Dest)
7200 JMPJump: PC ← ea(Dest) (This is the same as LEA PC,Dest and shares the same opcode.)
7400 ASHArithmetic shift: if Src < 0 then Reg ← Shift-right(Reg, -Src) else Reg ← Shift-left(Reg, Src)
7800 XCHExchange: Reg ↔ Src
7A00 ASHCArithmetic shift combined (32 bit): if Src < 0 then (Reg+1:Reg) ← Shift-right((Reg+1:Reg), -Src) else (Reg+1:Reg) ← Shift-left((Reg+1:Reg), Src)
7C00 MULMultiply: (Reg+1:Reg) ← Reg × Src
7E00 DIVDivide: Compute (Reg+1:Reg) ÷ Src; Reg ← quotient; Reg+1 ← remainder

Single-operand instructions

The high-order ten bits specify the operation to be performed, with bit 15 generally selecting byte versus word addressing. A single group of six bits specifies the operand as defined above.

15 6 5 3 2 0
Opcode Src/Dest Register
OpcodeMnemonicOperation
0BC0 SWABSwap bytes of word: Dest ← (Dest × 256) ∨ (Dest ÷ 256)
8BC0 SWADSwap digits of byte: Dest ← (Dest × 16) ∨ (Dest ÷ 16)
0840 CLRClear: Dest ← 0
8840 CLRB
0C00 COMComplement: Dest ← (-1 - Dest)
8C00 COMB
0C80 INCIncrement: Dest ← Dest + 1
8C80 INCB
0CC0 DECDecrement: Dest ← Dest − 1
8CC0 DECB
0C40 NEGNegate: Dest ← -Dest
8C40 NEGB
0B00 SETSet: Dest ← -1
8B00 SETB
0A80 TSTTest: Set-flags(Src)
8A80 TSTB
0A00 RORRotate right: Dest:Cflag ← Rotate-right(Dest:Cflag, 1)
8A00 RORB
0A40 ROLRotate left: Cflag:Dest ← Rotate-left(Cflag:Dest, 1)
8A40 ROLB
0B80 ASRArithmetic shift right: Dest ← Dest ÷ 2, sign preserved
8B80 ASRB
0AC0 ASLArithmetic shift left: Dest ← Dest × 2
8AC0 ASLB
8D80 ADCAdd carry: Dest ← Dest + Cflag
8DC0 SBCSubtract carry: Dest ← Dest - Cflag
0D00 IW2Increment word by 2: Dest ← Dest + 2
0DC0 TJMPTabled jump: PC ← PC + (Dest), PC ← PC + @PC
0D80 TCALLTabled call: -(SP) ← PC, PC ← PC + (Dest), PC ← PC + @PC
0D40 SXTSign extend: if N flag = 1 then Dest ← -1 else Dest ← 0
8D00 LSTSLoad processor status: PSW ← Dest
8D40 SSTSSave processor status: Dest ← PSW

Single-operand short immediate instructions

The high-order seven bits and bits 5 and 4 specify the operation to be performed. A single group of three bits specifies the register. A four bit count field contains a small immediate or a count. In all cases one is added to this field making the range 1 through 16.

15 9 8 6 5 4 3 0
Opcode Reg Op Count
OpcodeMnemonicOperation
0800 ADDIAdd immediate: Reg ← Reg + Count + 1
0810 SUBISubtract immediate: Reg ← Reg - Count - 1
0820 BICIBit clear immediate: Reg ← Reg ∧ (-1 - (Count+1))
0830 MOVIMove immediate: Reg ← Count + 1
8800 SSRRRight rotate multiple: Reg:C-flag ← Rotate-right(Reg:C-flag, Count+1)
8810 SSLRLeft rotate multiple: C-flag:Reg ← Rotate-left(C-flag:Reg, Count+1)
8820 SSRARight arithmetic shift multiple: Reg:C-flag ← Arithmetic-shift-right(Reg, Count+1)
8830 SSLALeft arithmetic shift multiple: C-flag:Reg ← Arithmetic-shift-left(Reg, Count+1)
8E00 SDRRDouble right rotate multiple (33 bit): Reg+1:Reg:C-flag ← Rotate-right(Reg+1:Reg:C-flag, Count+1)
8E10 SDLRDouble left rotate multiple (33 bit): C-flag:Reg+1:Reg ← Rotate-left(C-flag:Reg+1:Reg, Count+1)
8E20 SDRADouble right arithmetic shift multiple (32 bit): Reg:Reg+1:C-flag ← Arithmetic-shift-right(Reg:Reg+1, Count+1)
8E30 SDLADouble left arithmetic shift multiple (32 bit): C-flag:Reg:Reg+1 ← Arithmetic-shift-left(Reg:Reg+1, Count+1)

Floating point instructions

The high-order eight bits specify the operation to be performed. Two groups of four bits specify the source and destination addressing mode and register. If field I = 0, designated register contains the address of the operand, the equivalent of addressing mode (Rn). If field I = 1, designated register contains the address of the address of the operand, the equivalent of addressing mode @0(Rn).

15 8 7 6 4 3 2 0
Opcode I SReg I DReg
OpcodeMnemonicOperation
F000 FADDFloating add: Dest ← Dest + Src
F100 FSUBFloating subtract: Dest ← Dest - Src
F200 FMULFloating Multiply: Dest ← Dest × Src
F300 FDIVFloating Divide: Dest ← Dest ÷ Src
F400 FCMPFloating Compare: Dest - Src

Block transfer instructions

The high-order ten bits specify the operation to be performed. Two groups of three bits specify the source and destination registers. In all cases the source register contains the address of the first word or byte of memory to be moved, and the destination register contains the address of the first word or byte of memory to receive the data being moved. The number of words or bytes being moved is contained in R0 as a unsigned integer. The count ranges from 1–65536. These instructions are fully interruptible.

15 6 5 3 2 0
Opcode SReg DReg
OpcodeMnemonicOperation
0E00 MBWUMove block of words up: (DReg) ← (SReg), SReg ← SReg + 2, DReg ← DReg + 2, R0 ← R0 - 1, until R0 = 0
0E40 MBWDMove block of words down: (DReg) ← (SReg), SReg ← SReg - 2, DReg ← DReg - 2, R0 ← R0 - 1, until R0 = 0
0E80 MBBUMove block of bytes up: (DReg) ← (SReg), SReg ← SReg + 1, DReg ← DReg + 1, R0 ← R0 - 1, until R0 = 0
0EC0 MBBDMove block of bytes down: (DReg) ← (SReg), SReg ← SReg - 1, DReg ← DReg - 1, R0 ← R0 - 1, until R0 = 0
0F00 MBWAMove block of words to address: (DReg) ← (SReg), SReg ← SReg + 2, R0 ← R0 - 1, until R0 = 0
0F40 MBBAMove block of bytes to address: (DReg) ← (SReg), SReg ← SReg + 1, R0 ← R0 - 1, until R0 = 0
0F80 MABWMove address to block of words: (DReg) ← (SReg), DReg ← DReg + 2, R0 ← R0 - 1, until R0 = 0
0FC0 MABBMove address to block of bytes: (DReg) ← (SReg), DReg ← DReg + 1, R0 ← R0 - 1, until R0 = 0

Branch instructions

The high-order byte of the instruction specifies the operation. The low-order byte is a signed word offset relative to the current location of the program counter. This allows for forward and reverse branches in code. Maximum branch range is +128, -127 words from the branch op code.

In most branch instructions, whether the branch is taken is based on the state of the condition codes. A branch instruction is typically preceded by a two-operand CMP (compare) or BIT (bit test) or a one-operand TST (test) instruction. Arithmetic and logic instructions also set the condition codes. In contrast to Intel processors in the x86 architecture, MOV instructions set them too, so a branch instruction could be used to branch depending on whether the value moved was zero or negative.

15 8 7 0
Opcode Offset
OpcodeMnemonicCondition or Operation
0100 BRBranch always PC ← PC + (2 × Sign-extend(Offset))
0200 BNEBranch if not equal Z = 0
0300 BEQBranch if equal Z = 1
0400 BGEBranch if greater than or equal (N ⊻ V) = 0
0500 BLTBranch if less than (N ⊻ V) = 1
0600 BGTBranch if greater than (Z ∨ (N ⊻ V)) = 0
0700 BLEBranch if less than or equal (Z ∨ (N ⊻ V)) = 1
8000 BPLBranch if plus N = 0
8100 BMIBranch if minus N = 1
8200 BHIBranch if higher (C ∨ Z) = 0
8300 BLOSBranch if lower or same (C ∨ Z) = 1
8400 BVCBranch if overflow clear V = 0
8500 BVSBranch if overflow set V = 1
8600 BCC or BHISBranch if carry clear, or Branch if higher or same C = 0
8700 BCS or BLOBranch if carry set, or Branch if lower C = 1

The limited range of the branch instructions meant that as code grows, the target addresses of some branches may become unreachable. The programmer would change the one-word Bcc to the two-word JMP instruction. As JMP has no conditional forms, the programmer would change the Bcc to its opposite sense to branch around the JMP.

SOB (Subtract One and Branch) is another conditional branch instruction. The specified register is decremented by 1, and if the result is not zero, a reverse branch is taken based on the 6-bit word offset.

15 9 8 6 5 0
Opcode Reg Offset
OpcodeMnemonicOperation
7600 SOBSubtract One and Branch: Reg ← Reg - 1; if Reg ≠ 0 then PC ← PC - (2 × Offset)

Subroutine instructions

The JSR instruction can save any register on the stack. Programs that do not need this feature specify PC as the register (JSR PC, address) and the subroutine returns using RTN PC. If a routine were called with, for example, "JSR R4, address", then the old value of R4 would be saved on the top of the stack and the return address (just after JSR) would be in R4. This lets the routine gain access to values coded in-line by specifying (R4)+ or to in-line pointers by specifying @(R4)+. The autoincrementation moves past these data, to the point at which the caller's code resumes. Such a routine would have to specify RTN R4 to return to its caller.

The JSR PC,@(SP)+ form can be used to implement coroutines. Initially, the entry address of the coroutine is placed on the stack and from that point the JSR PC,@(SP)+ instruction is used for both the call and the return statements. The result of this JSR instruction is to exchange the contents of the PC and the top element of the stack, and so permit the two routines to swap control and resume operation where each was terminated by the previous swap.

15 9 8 6 5 3 2 0
Opcode Reg Src Register
OpcodeMnemonicOperation
7000 JSRJump to subroutine: -(SP) ← Reg; Reg ← PC; PC ← Src
15 3 2 0
Opcode Reg
OpcodeMnemonicOperation
0018 RTNReturn from subroutine: PC ← Reg; Reg ← (SP)+
0028 PRTNPop stack and return: SP ← SP + (2 × @SP), PC ← Reg; Reg ← (SP)+

Single register instructions

These instructions have a 13 bit opcode and a three bit register argument.

15 3 2 0
Opcode Reg
OpcodeMnemonicOperation
0010 IAKInterrupt acknowledge: Interrupt acknowledge state code, Reg ← Bus read
0020 MSKOMask out: (002E) ← Reg, Mask out state code

Implied parameter instructions

15 0
Opcode
OpcodeMnemonicOperation
0000 NOPNo operation: Do nothing
0001 RESETReset: Transmit reset pulse to I/O devices
0002 IENInterrupt enable: I2 ← 1
0003 IDSInterrupt disable: I2 ← 0
0004 HALTHalt: Executes the selected halt option
0005 XCTExecute single instruction: PC ← (SP)+, PS ← (SP)+, set trace flag, execute opcode, -(SP) ← PS, -(SP) ← PC, trace flag reset, If no error PC ← (0020) else PC ← (001E)
0006 BPTBreakpoint trap:-(SP) ← PS, -(SP) ← PC, PC ← (002C)
0007 WFIWait for interrupt: Enable interrupts (I2 ← 1). Bus activity ceases.
0008 RSVCReturn from supervisor call (B or C): REST, SP ← SP + 2, RTT
0009 RRTTRestore and return from trap: REST, RTT
000A SAVESave registers: -(SP) ← R5, -(SP) ← R4, -(SP) ← R3, -(SP) ← R2, -(SP) ← R1, -(SP) ← R0
000B SAVSSave status: SAVE, -(SP) ← (002E), (002E) ← (002E) ∨ Mask, MSKO, IEN. Note Mask is stored in the second word of instruction.
000C RESTRestore registers: R0 ← (SP)+, R1 ← (SP)+, R2 ← (SP)+, R3 ← (SP)+, R4 ← (SP)+, R5 ← (SP)+
000D RRTNRestore and return: REST, PC ← (SP)+
000E RSTSRestore status: (002E) ← (SP)+, MSKO, REST, RTT
000F RTTReturn from trap: PC ← (SP)+, PS ← (SP)+

Supervisor calls

These instructions are used to implement operating system (supervisor) calls. All have a six bit register argument. SVCB and SVCC are designed so an argument to the operating system can use most of the addressing modes supported by the native instruction set.

15 6 5 0
Opcode Arg
OpcodeMnemonicOperation
0040 SVCASupervisor Call A: -(SP) ← PS ,-(SP) ← PC, PC ← (0022) + Arg × 2, PC ← (PC)
0080 SVCBSupervisor Call B: TMPA ← SP, -(SP) ← PS, -(SP) ← PC, TMPB ← SP, -(SP) ← TMPA, -(SP) ← R5, -(SP) ← R4, -(SP) ← R3, -(SP) ← R2, -(SP) ← R1, -(SP) ← R0, R1 ← TMPB, R5 ← Arg × 2, PC ← (0024)
00C0 SVCCSupervisor Call C: TMPA ← SP, -(SP) ← PS, -(SP) ← PC, TMPB ← SP, -(SP) ← TMPA, -(SP) ← R5, -(SP) ← R4, -(SP) ← R3, -(SP) ← R2, -(SP) ← R1, -(SP) ← R0, R1 ← TMPB, R5 ← Arg × 2, PC ← (0026)

Condition-code operations

15 4 3 2 1 0
Opcode N Z V C
OpcodeMnemonicOperation
0030 LCCLoad condition codes: Load according to N, Z, V, C bits

The four condition codes in the processor status word (PSW) are

  • N indicating a negative value
  • Z indicating a zero (equal) condition
  • V indicating an overflow condition, and
  • C indicating a carry condition.

Reserved low-memory locations

Memory locations between 0000 and 003F have fixed functions defined by the processor. All addresses below are word addresses.[9]

Vector Condition
0000-0010 R0 - R5, SP, PC, and PS for power up/halt options
0012 bus error PC
0014 nonvectored interrupt power fail PC
0016 power up/halt option power restore PC
0018 parity error PC
001A reserved op code PC
001C illegal op code format PC
001E XCT error PC
0020 XCT trace PC
0022 SVCA table address
0024 SVCB PC
0026 SVCC PC
0028 vectored interrupt (I0) table address
002A nonvectored interrupt (I1) PC
002C BPT PC
002E I/O priority interrupt mask
0030-003C Floating point scratchpad
003E Floating point error PC

Performance

WD16 processor speed varies by clock speed, memory configuration, op code, and addressing modes. Instruction timing has up to three components, execute/fetch of the instruction itself and access time for the source and the destination. The last two components depend on the addressing mode. For example, at 3.3 MHz, an instruction of the form ADD x(Rm),y(Rn) has a fetch/execute time of 3.3 microseconds plus source time of 2.7 microseconds and destination time of 3.0 microseconds, for a total instruction time of 9.0 microseconds. The register-to-register ADD Rm,Rn executes in 3.3 microseconds. Floating point is significantly slower. A single-and-a-half precision (48 bit) floating add instruction typically ranges from 54 to 126 microseconds. The WD16's precision is a compromise between traditional single and double precision floats.[9]

For contrast, the fastest PDP-11 computer at the time was the PDP-11/70. An instruction of the form ADD x(Rm),y(Rn) has a fetch/execute time of 1.35 microseconds plus source and destination times of 0.6 microseconds each, for a total instruction time of 2.55 microseconds. Any case where addressed memory was not in the cache adds 1.02 microseconds. The register-to-register ADD Rm,Rn could execute from the cache in 0.3 microseconds.[13] A single-precision floating add instruction executed by the FP11-C co-processor could range from 0.9 to 2.5 microseconds plus time to fetch the operands which could range up to 4.2 microseconds.[14]

The WD16 block transfer instructions approximately double the speed of moves and block I/O. A word moved with MOV (R1)+,(R2)+, SOB R0,loop instructions takes 9.6 microseconds per iteration. The MBWU R1,R2 equivalent takes 4.8 microseconds per iteration.

The Association of Computer Users performed a series of benchmarks on an AM-100T-based system costing $35,680 (equivalent to $126,725 in 2022). They found that their CPU-bound benchmark executed in 31.4 seconds on the AM-100T compared to 218 seconds for the average single user system in the $15,000 to $25,000 price range.[15] In a group of multi-user computers priced between $25,000 and $50,000, the AM-100T was in the "upper third" for speed.[16]

The Creative Computing Benchmark of May 1984 placed the WD16 (in the AM-100T application) as number 34 out of 183 machines tested. The elapsed time was 10 seconds, compared to 24 seconds for the IBM PC.[17]

Emulator

Virtual Alpha Micro is an open source WD16 emulator. Written in C, it emulates the WD16 processor and the Alpha Micro AM-100 hardware environment. The author claims it runs on Linux (including Raspberry Pi), Windows, and Macintosh desktops, though no binaries are provided. It will run the Alpha Micro Operating System (AMOS) and all associated programs. In 2002, Alpha Micro granted limited permission to distribute AMOS 4.x or 5.0 binaries including the manuals for hobby use only.[18]

See also

PDP-11 architecture

References

  1. Flystra, Daniel (April 1977). "New Hobbyist System Supports Timesharing" (PDF). Byte. 2 (4): 142–144. Retrieved 21 November 2022.
  2. Wilcox, Dick (January 1977). "A PDP-11-Like 16-Bit Micro for the S-100 Bus". Dr. Dobbs's Journal of Computer Calisthenics and Orthodontia. 2 (1): 3–7. Retrieved 14 June 2022.
  3. MCP-1600 Microprocessor Users Manual (PDF). Western Digital. 1975. Retrieved 28 April 2022.
  4. Stan, Viet (May 1983). "The Battle of the Disk Operating Systems" (PDF). Computers & Electronics. 21 (5): 19. Retrieved 8 June 2023.
  5. Craig, John (April 1977). "Around the Industry" (PDF). Kilobaud: The Small Comuter Magazine (4): 10. Retrieved 23 November 2022.
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