This program patches the MTERM machine code package, a terminal emulator, by poking specific byte values into three memory regions starting at addresses 59173, 59188, and 58618. The patch data consists of Z80 machine code bytes stored in DATA statements and read sequentially via a RESTORE/READ/POKE loop driven by subroutine calls at line 4500. The program verifies user readiness before proceeding and prints each address and its new value as confirmation. After patching, the user is instructed to save the modified MTERM back to tape.
Program Analysis
Program Structure
The program is divided into two functional segments. Lines 100–130 form a short guard that asks whether the MTERM code is already loaded in memory; a “yes” answer jumps to the patching routine at line 4000, while any other answer prints a message and halts. The main body from 4000 onward performs the patch, displays author contact information, and holds the DATA table and the READ/POKE subroutine.
Patching Mechanism
Three memory ranges are targeted in sequence:
59173–59175(3 bytes) — patched in the loop at lines4010–403059188–59190(3 bytes) — patched in the loop at lines4040–406058618–58833(216 bytes) — patched in the loop at lines4070–4090
A RESTORE 4100 at line 4000 resets the DATA pointer to the start of the patch table before any reading begins, ensuring the bytes are consumed in the correct order regardless of prior program state.
READ/POKE Subroutine
The subroutine at lines 4510–4530 is minimal but effective: it reads one byte from the DATA stream into variable a, POKEs it to the current loop address f, prints both the address and the value now held there via PEEK f as a live verification step, then returns. Printing every address/value pair makes the patching process fully visible to the user, which also serves as a rudimentary checksum substitute.
Z80 Machine Code Content
The DATA bytes from line 4100 onward encode Z80 instructions. Notable sequences include:
195(JP nn) — unconditional jumps at the very start of each small patch block237, 75(ED 4B=LD BC,(nn)) and237, 66(ED 42=SBC HL,BC) — extended Z80 instructions219, 254/219, 127(IN A,(254)/IN A,(127)) — port reads, consistent with keyboard and serial I/O in a terminal context211, 127(OUT (127),A) — port 127 output, typical of RS-232 serial hardware interfaces used with MTERM- A run of 24 zero bytes at line
4134suggesting a buffer or padding area within the replaced routine
Data Layout
| DATA lines | Approx. byte count | Purpose |
|---|---|---|
4100–4102 | 6 | First two small patch blocks (JP targets) |
4104–4184 | ~210 | Main replacement routine (58618–58833) |
Key BASIC Idioms
RESTORE 4100before anyREADto anchor the DATA pointer explicitly — good practice when multiple loops consume the same DATA stream sequentially.- Using
FOR f = addr1 TO addr2 : GO SUB 4500 : NEXT frather than inline POKE keeps the code compact and reuses a single subroutine for all three address ranges. - The subroutine at line
4500is called viaGO SUB 4500, but the actual code starts at line4510; line4500does not exist. This is intentional — the interpreter will execute the next available line (4510) without error, a standard space-saving trick.
Anomalies and Notes
The INPUT prompt at line 100 only branches on the exact string "yes" (lowercase); any variation such as "YES" or "Yes" will fall through to the STOP at line 130. There is no explicit error handling if the DATA runs short relative to the address range — a mismatch would produce an “Out of DATA” error mid-patch, potentially leaving MTERM in a partially modified state. The author’s mailing address embedded in lines 4094–4096 identifies this as a distributed fix meant to be typed in by users who contacted the author.
Content
Image Gallery
Source Code
100 INPUT "DO YOU HAVE MTERM CODE LOADED?"'" yes/no ";a$
110 IF a$="yes" THEN GO TO 4000
120 PRINT "PLEASE LOAD MTERM CODE AND RUN"
130 STOP
4000 RESTORE 4100
4010 FOR f=59173 TO 59175
4020 GO SUB 4500
4030 NEXT f
4040 FOR f=59188 TO 59190
4050 GO SUB 4500
4060 NEXT f
4070 FOR f=58618 TO 58833
4080 GO SUB 4500
4090 NEXT f
4092 PRINT ''''''"SAVE YOUR ALTERED MTERM ON TAPE"
4093 PRINT '''
4094 PRINT "DAVE SCHOENWETTER"
4095 PRINT "1335 FARM TO MARKET RD."
4096 PRINT "ENDWELL, NEW YORK, 13760"
4099 PRINT '''"all done": STOP
4100 DATA 195,126,229
4102 DATA 195,178,229
4104 DATA 42,232,238,237
4106 DATA 75,236,238,3
4108 DATA 125,185,32,7
4110 DATA 124,184,32,3
4112 DATA 195,80,229,62
4114 DATA 127,219,254,31
4116 DATA 210,80,229,219
4118 DATA 127,230,16,32
4120 DATA 242,126,254,10
4122 DATA 32,2,62,0
4124 DATA 254,12,32,2
4126 DATA 62,0,205,48
4128 DATA 229,35,195,2
4130 DATA 229,0,211,127
4132 DATA 0,219,127,201
4134 DATA 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
4136 DATA 0,0,62,13
4138 DATA 205,48,229,62
4140 DATA 13,205,48,229
4142 DATA 62,13,205,48
4144 DATA 229,33,117,239
4146 DATA 54,13,6,32
4148 DATA 43,54,32,16
4150 DATA 251,125,50,231
4152 DATA 238,201,33,195
4154 DATA 229,126,167,32
4156 DATA 4,62,35,119
4158 DATA 201,175,24,251
4160 DATA 203,79,194,40
4162 DATA 231,245,197,229
4164 DATA 42,176,229,237
4166 DATA 75,174,229,237
4168 DATA 66,32,4,225
4170 DATA 193,241,201,219
4172 DATA 127,230,16,32
4174 DATA 246,10,205,48
4176 DATA 229,3,62,96
4178 DATA 184,32,3,1
4180 DATA 0,94,237,67
4182 DATA 174,229,24,227
4184 DATA 0,94,0,94
4186 DATA 229,219,115,254
4188 DATA 12,40,4,254
4190 DATA 10,32,1,175
4192 DATA 245,42,176,229
4194 DATA 119,35,62,96
4196 DATA 188,32,3,33
4198 DATA 0,94,34,176
4200 DATA 229,241,225,201
4510 READ a: POKE f,a
4520 PRINT f,PEEK f
4530 RETURN Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.