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What's included here... There is a program listing, a detailed set of instructions and descriptions, and a program example.
The H67 is an HP-41/HP-42 program that simulates the CPU, registers and functions of a hypothetical computer that can be programmed in its own machine language. Note: I have re-written the original HP-67 program so that it runs on the HP-41/HP-42. If you really want the original HP-67 program, email me.
What is the use of writing and playing with such a program? Simulators exist for two useful reasons: 1) In the real world, it allows computer programmers to write and test software for a machine that hasn't been built yet, and 2) they allow people who don't usually have the opportunity to work at the machine level to do so.
This simulator will be very helpful to someone trying to learn how computers work at a very low level, i.e., things like given its architecture, what is the best way of implementing subroutines and passing parameters, etc. The ability to single-step execution is a great help.
H67 Description:
1) The basic unit of memory in the simulator is a 3 digit word or cell.
Three words of memory are stored in an HP-41/HP-42 memory register.
2) Memory capacity is 50 words.
3) The H67 has 5 CPU registers:
HP-41/42 Memory H67 Register
10 rA - Accumulator
11 rB - Index, Work Register
12 rC - Work Register
13 PC - Program Counter
14 CC - Condition Code
4) Addressing Modes are : 1) Immediate (Operand is part of the
instruction itself), 2) Register (Operand is in a register), 3) Direct
and 4) Indexed are both described later.
5) Instruction Set. These will be briefly listed here and described in
more detail later.
HLT 000 Halt, Display rA XCA 020 Exchange rC and rA L 1aa Load ST 2aa Store A 3aa Add S 4aa Subtract B 5aa Branch BC 6aa,aa Branch on condition I/D 7mr Increment/Decrement register LBI 8ii Load rB Immediate T 9rr Transfer Register
Register H67 Words 16 00 01 02 17 03 04 05 18 06 07 08 .... 31 45 46 47 32 48 49
A value in the range 00..49 is a direct address. Thus a 116 instruction means to load into the rA register the value located at location 16.
A value in the range 50..99 is an indexed address, whose effective address is determined as EA = (AA-50) + rB, that is, the effective address is found by subtracting 50 and adding the contents of rB. Thus, if rB = 2, then 156 would mean to load rA from the value at location 56- 50 + 2 or location 8.
Condition Codes: The condition code (CC) is set after any operation that changes the contents of a register. The CC reflects the contents of that register. If the register is less than 0, the CC is less than 0. If the register is 0, the CC is 0. If the register is greater than 0, the CC is greater than 0. The condition code is used in conjunction with the Branch on Condition instruction.
HLT, 000, Halt and Display rA: Halts and displays the contents of rA. At the time of the halt, the PC will be pointing to the next instruction and the CC will be unchanged from its last setting. If the operator keys in a value and then presses R/S, the H67 will load the entered value into rA, set the CC and then continue execution.
XCA, 020, Exchange rC and rA: Exchanges rC and rA, with the CC set to reflect the new rA contents (the previous rC contents).
L, 1aa, Load rA: Loads the rA register from the effective address aa, which may be direct or indexed.
ST, 2aa, Store rA: Stores the contents of the rA register into memory at the effective address aa, which may be direct or indexed. Note: The CC is not altered.
A, 3aa, Add to rA: Adds the contents of memory at effective address aa to the contents of rA. Address aa may be direct or indexed.
A, 4aa, Subtract from rA: Subtracts the contents of memory at effective address aa from the contents of rA leaving the result in rA. Address aa may be direct or indexed.
B, 5aa, Branch to aa: Branch to the effective address aa and continue execution there. Address aa may be direct or indexed. Note: The CC is not altered.
BC, 6aa,bb, Branch on Condition: This is a two-word instruction. The first word contains the opcode '6' and the address aa. The second word contains the address bb. aa and bb may be direct or indexed, independently of each other. If the CC is less than zero, the H67 branches to the effective address aa. If the CC is equal to zero, the H67 branches to effective address bb. If the CC is greater than zero, execution continues with the next sequential instruction. Note: The CC is not altered.
I or D, 7mr, Increment or Decrement Register: Increments or
decrements an H67 register. Fields m and r specify the mode and the
register according to the list below:
m = 2 = increment
m = 0 = decrement
r = 0 = rA
r = 1 = rB
r = 2 = rC
LBI, 8ii, Load rB Immediate: Loads rB with the two digit value ii.
T, 9rr, Transfer Register: This instruction consists of the
opcode 9 followed by a source register r, followed by the destination
register r. The source register contents are transfered to the
destination register contents by this instruction. The values for r
include:
r = 0 = rA
r = 1 = rB
r = 2 = rC
For example, 921 transfers the contents of rB to rC.
I have used the MUL routine provided below to actually write a factorial program using this program. It worked. Just don't ask me how long it took.
Does this listing bring back memories for some of you out there? It does for me. I'm reminded of carrying stackes of punched cards when I took Assembler!
REG LOC CONTENTS LABEL OP ADDRESS COMMENTS
; SAMPLE PROGRAM TO COMPUTE (6*8)-(3*4) AND TO
; ILLUSTRATE SUBROUTINES AND PASSING PARAMETERS
;
16 00 802 PGM: LBI PARMS ;B=ADDR(PARMS)
01 513 B MUL ;CALL MUL
02 003 PARMS: CON 3
17 03 004 CON 4
04 000 ANS1: RES 1 ;WILL BE 3*4
05 807 LBI .+2 ;NEW PARMS
18 06 513 B MUL ;CALL MUL
07 006 PARMS2: CON 6
08 008 CON 8
19 09 000 ANS2: RES 1 ;WILL BE 6*8
10 902 T A,C ;C=A=(6*8)
11 404 S ANS1 ;A=(6*8)-(3*4)
20 12 000 HLT ;DISPLAY A
;
; MUL - MULTIPLICATION ROUTINE
; CALL MUL BY: LBI .+2
; B MUL
; N CON V1
; S CON V2
; T RES 1
; WHERE N < S. PRODUCT IS ALSO IN REGISTER A.
; EXECUTION RESUMES AT THE INSTRUCTION FOLLOWING
; THE PARAMETER LIST
13 150 MUL: L 0,B ;A=MULTIPLIER
14 020 XCA
21 15 151 L 1,B ;MULTIPLICAND
16 702 MLOOP: D C ;IF CC = 0 THEN DONE
17 623 BC ERR;DONE ;ERR - CAN'T BE <0
22 18 021
19 351 A 1,B ;FORMING PRODUCT
20 516 B MLOOP
23 21 252 DONE: ST 2,B ;STORE PRODUCT
22 553 B 3,B ;RETURN
23 000 ERR: HLT ;ERROR - HALT
24 24 000 END PGM
To load this program, store the following values into these HP-41/HP-42
memories. The spaces between the 3 digits are to illustrate the H67
instructions contained.
R16 - 0.802 513 003
R17 - 0.004 000 807
R18 - 0.513 006 008
R19 - 0.000 902 404
R20 - 0.000 150 020
R21 - 0.151 702 623
R22 - 0.021 351 516
R23 - 0.252 553 000
R24 - 0.0
If you actually want to play with this simulator, I suggest you create a separate program that will store the values above into the proper memories to restore the original program. Run that loading program AFTER you have booted the H67.
LBL B - Pressing this key will boot the H67, clearing the program
registers and setting up certain constants. This is only necessary the
first time you run the program.
LBL D - Pressing this key will put the H67 into single-step mode.
LBL C - Pressing this key will put the H67 back into normal processing
mode.
LBL E - Pressing this key will begin program execution. Be patient.
Prior to running the program, be sure to load rA, rB, rC, PC, or CC as needed. Common mistake Don't boot by pressing the B key AFTER you have loaded the program starting in memory 16! You will erase your program!
While running programs in single-step mode (a good way to debug what you have written), the H67 will stop and display the PC of the instruction about to be executed, allowing you to follow the program flow. When it is stopped like this, you may view any registers, etc., as long as you return the value of the PC to the display prior to pressing R/S to continue execution.
Line Instruction 01 LBL "H67" 02 LBL E 03 RCL 13 04 FS? 01 05 STOP 06 XEQ 15 07 RCL 06 08 1 09 + 10 STO 13 11 RDN 12 100 13 STO 07 14 / 15 STO 15 16 FRC 17 RCL 07 18 * 19 GTO IND 15 20 LBL 15 21 XEQ 14 22 STO 06 23 3 24 / 25 INT 26 STO 05 27 RCL 08 28 + 29 STO 04 30 STO 15 31 RCL 06 32 RCL 05 33 3 34 * 35 - 36 STO 00 37 RCL IND 15 38 RCL 09 39 * 40 INT 41 STO 03 42 LASTX 43 FRC 44 RCL 09 45 * 46 INT 47 STO 02 48 LASTX 49 FRC 50 RCL 09 51 * 52 INT 53 STO 01 54 3 55 RCL 00 56 - 57 STO 15 58 FS?C 02 59 GTO 11 60 RCL IND 15 61 RTN 62 LBL 11 63 RCL 07 64 STO IND 15 65 RCL 04 66 STO 15 67 RCL 03 68 RCL 02 69 RCL 01 70 RCL 09 71 / 72 + 73 RCL 09 74 / 75 + 76 RCL 09 77 / 78 STO IND 15 79 RTN 80 LBL 14 81 50 82 X > Y? 83 GTO 14 84 - 85 RCL 11 86 + 87 RTN 88 LBL 14 89 RDN 90 RTN 91 LBL 01 92 XEQ 15 93 STO 10 94 STO 14 95 GTO E 96 LBL 02 97 RCL 10 98 STO 07 99 RDN 100 SF 02 101 XEQ 15 102 GTO E 103 LBL 09 104 10 105 / 106 LASTX 107 STO 07 108 + 109 STO 15 110 RCL IND 15 111 X <> 15 112 FRC 113 10 114 * 115 RCL 07 116 + 117 X <> 15 118 STO IND 15 119 STO 14 120 GTO E 121 LBL 07 122 10 123 / 124 STO 07 125 INT 126 1 127 - 128 RCL 07 129 FRC 130 10 131 * 132 LASTX 133 + 134 STO 15 135 RDN 136 STO+ IND 15 137 RCL IND 15 138 STO 14 139 GTO E 140 LBL 06 141 RCL 14 142 X = 0? 143 GTO 06 144 X > 0? 145 GTO 12 146 RDN 147 XEQ 14 148 GTO 13 149 LBL 06 150 RCL 13 151 XEQ 15 152 XEQ 14 153 GTO 13 154 LBL 12 155 RCL 13 156 1 157 + 158 LBL 13 159 STO 13 160 GTO E 161 LBL 03 162 XEQ 15 163 RCL 10 164 + 165 STO 10 166 STO 14 167 GTO E 168 LBL 08 169 STO 11 170 STO 14 171 GTO E 172 LBL 05 173 XEQ 14 174 STO 13 175 GTO E 176 LBL 04 177 XEQ 15 178 RCL 10 179 X <> Y 180 - 181 STO 10 182 STO 14 183 GTO E 184 LBL 00 185 X NE 0? 186 GTO 00 187 RCL 10 188 STOP 189 STO 10 190 STO 14 191 GTO E 192 LBL 00 193 RCL 12 194 RCL 10 195 STO 12 196 RDN 197 STO 10 198 STO 14 199 GTO E 200 LBL C 201 CF 01 202 GTO E 203 LBL D 204 SF 01 205 GTO E 206 LBL B 207 0.015 208 ENTER 209 0 210 LBL 17 211 STO IND Y 212 ISG Y 213 GTO 17 214 16 215 STO 08 216 1000 217 STO 09 218 CF 01 219 CF 02 220 CLST 221 ENDThat's it. Enjoy and Learn!